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PRACTICAL  OIL  GEOLOGY 


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PRACTICAL  OIL 
GEOLOGY 

THE  APPLICATION  OF  GEOLOGY  TO 
OIL  FIELD  PROBLEMS 


BY 
DORSEY  HAGER 

PETROLEUM   GEOLOGIST  AND   ENGINEEB 


SECOND  EDITION 

THOROUGHLY  REVISED  AND  ENLARGED 
SECOND  IMPRESSION 


McGRAW-HILL  BOOK  COMPANY,  INC, 

239  WEST  39TH  STREET.     NEW  YORK 


LONDON:  HILL  PUBLISHING  CO.,  LTD. 

6  &  8  BOUVERIE  ST.,  E.G. 

1916 


COPYRIGHT  1915,  1916,  BY  THE 

McGRAW-HlLL   B.OC/5    CpiVlipA.NY,    INC. 


THE» MAPLE.  PRESS*  YORK.  PA 


Go  tbc 

PRACTICAL   OIL   MAN   OF    AMERICA,   WITH   THE   HOPE   THAT 
THIS  BOOK  WILL  BRING  HIM  TO  A  BETTER  UNDER- 
STANDING   OF     THE     RELATION    OF    THE 
GEOLOGIST  TO  THE    PETROLEUM 
INDUSTRY 


M100495 


PREFACE  TO  SECOND  EDITION 

In  making  a  revision  of  the  first  edition  the  author  wishes 
especially  to  thank  Messrs.  Johnson  and  Huntley  of  Pittsburgh, 
and  Fred  Pack  on  the  United  States  Geological  Survey,  for  their 
published  criticisms  of  the  first  edition  of  " Practical  Oil  Geology." 

He  also  wishes  to  thank  his  many  friends  and  fellow  geologists 
and  engineers,  especially  Mowry  Bates  and  J.  W.  Lewis  of 
Tulsa,  for  their  suggestions.  The  industry  is  so  extensive  and 
the  work  of  the  geologist  is  so  broad  that  the  writer  feels  he  has 
but  touched  on  the  many  uses  of  geology  in  oil-field  practice,  but 
he  hopes  the  book  will  prove  useful.  Certainly  it  has  already 
more  than  fulfilled  the  writer's  expectations  for  which  he  is  deeply 
appreciative  to  the  reading  public. 

DORSET  HAGER. 
TULSA,  OKLA. 
November,  1916. 


vii 


PREFACE  TO  FIRST  EDITION 

In  the  preparation  of  this  book  the  author  aimed  to  furnish 
the  oil  man  with  a  clear,  concise,  and  practical  work  on  the 
occurrence  of  oil,  and  its  geology.  There  are  several  works  on 
petroleum  but  none  of  them  is  in  handbook  form.  Three  of 
the  works  are  by  English  authors  who  give  the  practice  of  the 
East  Indian  and  the  Russian  oil  fields  rather  than  that  of  America. 
However  the  elements  of  oil-field  geology  are  the  same  the  world 
over,  though  the  best  chances  for  study  are  afforded  by  develop- 
ments in  America.  It  seemed  more  than  fitting,  therefore,  that 
American  oil  men  should  have  a  book  treating  more  particularly 
of  American  methods.  As  the  author  has  gained  his  experience 
in  these  fields  it  necessarily  follows  that  he  gives  the  Ameri- 
can viewpoint,  which  will  perhaps  be  a  just  basis  for  criticism 
by  those  who  have  had  a  world-wide  experience.  The  author 
has,  however,  drawn  the  data  for  this  book  from  European  as 
well  as  American  sources  and  hopes  thus  somewhat  to  overcome  a 
natural  bias. 

The  material  in  this  book  is  derived  from  the  following  sources : 

(1)  The  standard  text  books  on  general  geology  such  as  those 
by  Geikie,  Le  Conte,  Chamberlin  and  Salisbury,  and  Kemp. 

(2)  The  bulletins  of  the  U.  S.  Geological  Survey,  the  technical 
papers  of  the  U.  S.  Bureau  of  Mines  and  the  bulletins  of  the 
California,  Oklahoma,  Illinois,  Louisiana,  Pennsylvania  and  Ohio 
geological  surveys. 

(3)  The  articles  appearing  in  the  numerous  technical  journals 
on  mining  and  on  oil,  especially  papers  by  Lakes,  Clapp,  Gordon 
Sur,  Lee  Hager,  Breger,  Arnold,  Garfias,  Dumble  and  others. 

(4)  The  following  English  books:  "Petroleum  Mining"    by 
A.  Beeby  Thompson,  "Petroleum  and  Its  Sources"  by  Sir  Bover- 
ton  Redwood,  and  "Oil  Finding"  by  E.  H.  Cunningham  Craig. 

ix 


x  PREFACE 

(5)  The  catalogues  of  several  oil-well  supply  companies. 

The  author  is  also  greatly  indebted  to  his  good  friends  among 
the  operators  and  drillers  for  many  valuable  suggestions,  and 
for  their  assistance  in  helping  him  to  obtain  facts. 

Thanks  are  also  given  to  Messrs.  M.  J.  Munn,  J.  H.  Jenkins, 
Fohs  and  Gardner,  R.  A.  Conkling,  E.  D.  Bloesch,  Frank  But- 
tram,  E.  Thomas,  Valerius,  McNutt  and  Hughes,  and  A.  T. 
Patrick  for  their  kindness  in  affording  suggestions  and  additions 
to  make  the  work  more  complete. 

Special  recognition  is  due  to  H.  Foster  Bain,  and  to  Leon  Pep- 
perberg  for  their  criticisms  and  suggestions,  and  to  F.  J.  Basedow 
of  Adelaide,  Australia,  for  his  assistance  in  correcting  manuscript. 

As  is  the  case  in  all  sciences,  there  is  much  valuable  material 
which  it  is  difficult  to  trace  and  to  credit  to  the  originators. 
The  author  has  made  free  use  of  this  knowledge,  for  the  facts 
it  presents  are  among  the  most  valuable  we  possess. 

DORSET  HAGER. 
TULSA,  OKLA. 
April,  1915. 


CONTENTS 

PAGE 

PREFACE  TO  SECOND  EDITION vii 

FOREWORD , xiii 

CHAPTER 

I.  Petroleum — Its  Origin  and  Accumulation 1 

II.  Petroleum — Physical  and  Chemical  Properties 14 

III.  Stratigraphy 30 

IV.  Structural  Geology 48 

V.  Prospecting  and  Mapping 69 

VI.  Locating  Drill-Hole  Sites 94 

VII.  Factors  in  Oil- Well  Drilling 108 

VIII.  Factors  in  Oil  Production 132 

IX.  Water,  the  Enemy  of  the  Petroleum  Industry 151 

X.  Natural  Gas 165 

XI.  Cautions 174 

INDEX  ....                                       175 


XI 


FOREWORD 

Oil  Geology — Applied  Common  Sense 

There  is  at  present  a  rather  vague  idea  in  the  minds  of  many 
men  as  to  just  what  constitutes  an  oil  geologist.  Some  people 
associate  him  with  the  " crooked  stick"  or  " peach  tree  twig" 
men,  others  think  he  uses  some  hocus-pocus,  and  as  yet  com- 
paratively few  of  the  operators  see  the  geologist  as  a  clean-cut, 
clear-thinking  engineer,  who  is  just  as  much  an  expert  in  his  line 
as  is  the  driller  or  railroad  surveyor. 

The  geologist  simply  uses  engineering  methods  in  arriving  at 
results.  Engineering  instruments  such  as  transits,  levels,  barom- 
eters, alidades  and  plane  tables  are  employed,  all  of  which  re- 
quire a  mind  trained  in  mathematics  for  their  accurate  use.  The 
pick  of  the  geologist,  the  test-tube  and  the  chloroform  bottle 
are  also  his  working  tools.  Added  to  the  above  instruments, 
in  fact  of  primary  importance,  is  a  mind  trained  in  the  reading 
of  surface  forms  (topography) ,  a  knowledge  of  the  ages  of  forma- 
tions and  their  means  of  identification  (stratigraphy),  a  knowl- 
edge of  the  various  folded  structures  that  are  important  as  oil 
reservoirs,  and  above  all,  the  ability  to  recognize  such  folds  in  the 
field. 

By  studying  rock  exposure  at  the  surface,  by  using  drill  hole 
records,  either  of  water  or  oil  tests,  by  readings  in  mines,  etc., 
the  geologist  arrives  at  his  conclusions.  Of  course  where  all 
exposures  are  covered  up,  and  no  well  records  exist,  the  geologist 
is  "up  a  stump,"  and  can  only  say,  "I  do  not  know." 

The  geologist  knows  from  the  history  of  proven  oil  fields  that 
the  surface  folding  is  generally  an  index  to  underground  condi- 
tions. Enough  holes  "have  been  drilled,  enough  well  logs  have 
been  plotted  to  prove  this  very  important  point,  and  it  is  upon 

xui 


xiv  FOREWORD 

this  fact  that  the  science  of  oil  geology  is  based.  Recognition 
of  the  fact  that  underground  folding  is  reflected  at  the  surface 
and  that  such  surface  folding  can  generally  be  seen  meant  the 
beginning  of  a  new  era  for  the  oil  man. 

The  work  of  the  oil  geologist  really  simplifies  itself  into  the 
problem  of  finding  folds.  When  he  has  found  this  folding,  then 
he  has  a  basis  upon  which  to  work.  The  following  points  must 
be  emphasized :  First,  That  all  folds  do  not  carry  oil ;  second,  a 
geologist  cannot  tell  whether  a  fold  will  carry  oil,  unless  wells 
are  already  drilled  upon  it.  A  geologist  does  know,  however, 
that  the  majority  of  folds,  within  certain  defined  limits,  do  carry 
oil,  and  he  can  reason  from  this  that  the  chances  are  in  favor  of 
a  well-defined  fold  being  productive,  if  it  occurs  within  certain 
boundaries. 

In  the  last  few  years  sand  conditions  were  found  to  play  an 
important  part  in  oil  pools,  though  not  as  important  as  folding. 
Also  many  of  the  largest  wells  in  the  world  are  on  faults  and 
not  connected  in  any  way  with  folding. 

The  work  of  the  geologist  does  not  end  with  outlining  pro- 
spective oil  lands.  His  province  extends  into  the  fiejd  of  drilling 
and  of  actual  oil-field  development.  To  limit  geology  to  pro- 
spective territory  alone  is  a  great  loss,  for  as  one  will  find  from 
the  following  pages,  there  is  a  wide  and  varied  application  of 
geology  to  the  needs  of  the  oil  men  in  nearly  every  phase  of  oil- 
field work. 


PRACTICAL  OIL  GEOLOGY 

CHAPTER  I 
PETROLEUM— ITS    ORIGIN    AND    ACCUMULATION 

Much  has  been  written  about  the  conditions  under  which  oil  is 
found  in  nature  (the  structure  favorable  for  the  accumulation  of 
petroleum,  the  age  of  the  formations  in  which  oil  occurs,  and  the 
technology  of  drilling,  producing,  and  marketing  petroleum), 
but  as  yet  little  positive  knowledge  regarding  the  origin  of  oil 
has  appeared  in  print  although  the  subject  has  been  discussed 
very  fully  from  a  theoretical  viewpoint  by  many  geologists,  chem- 
ists and  engineers.  Some  of  the  many  theories  are  discussed 
below. 

CLASSIFICATION  OF  THEORIES 

Theories  pertaining  to  the  origin  of  petroleum  may  be  classified 
under  three  main  divisions  as  follows: 

(A)  Inorganic  theories;  (B)  Organic  theories;  and  (C)  Combi- 
nations of  Inorganic  and  Organic  theories. 

Inorganic  Theories. — By  inorganic  is  meant  any  chemical  re- 
actions that  take  place  without  the  aid  of  living  organisms. 
There  are  three  principal  inorganic  theories:  (1)  The  carbide 
theory;  (2)  the  limestone,  gypsum,  and  hot  water  theory;  and 
(3)  the  volcanic  theory. 

1.  The  carbide  theory  is  based  upon  the  fact  that  in  the  chemical 
laboratory  carbides  of  calcium,  iron,  and  several  other  elements 
give  hydrocarbon  products  when  in  contact  with  water.  It  is 
assumed  that  great  quantities  of  calcium,  aluminum,  iron  and 
other  similar  carbides  exist  deep  underground  and  that  the  action 

1 


2  PRACTICAL  OIL  GEOLOGY 

of  hot  water  upon  these  carbides  forms  liquid  and  gaseous  hydro- 
carbon .compounds, .that  rise  upward  through  fissures  and  other 
vents  in  the  .earth,  and,  collect  in  the  sedimentary  beds  above. 
This  theory  Jias  been  strongly  supported  by  some  able  chemists 
but  it  Is  not  Advocated  by  .many  geologists. 

2.  The  limestone,  gypsum,  and  hot  water  theory  is  advocated  by 
some  writers.     According  to  this  theory  the  action  of  heated 
water  upon  limestone  (CaCO3)  and  gypsum  (CaSO4),  which  in 
nature  are  closely  associated,  give  as  products  the  constituents  of 
petroleum.     The  exact  chemical  processes  have  not  been  fully 
explained,  but  it  is  certain  that  limestone,  gypsum  and  water 
contain  all  the  necessary  elements  for  the  production  of  petro- 
leum.    Under  certain  conditions  of  heat  and  pressure,  it  is  not 
impossible  that  oil  may  be  formed  as  thus  postulated. 

3.  The  volcanic  theory  is  based  upon  the  fact  that  gases  given  off 
from  some  volcanoes  carry  small  percentages  of  hydrocarbons. 
These  gases  are  supposedly  of  deep-seated  origin,  and  carry  the 
products  of  chemical  reactions  that  occur  in  the  earth.     It  is 
assumed  that  the  gases  are  condensed  before  reaching  the  surface 
by  coming  in  contact  with  cooler  formations  near  the  surface 
and  thus  form  petroleum.     As  a  laboratory  theory  the  volcanic 
idea  is  plausible  but  it  by  no  means  explains  most  of  the  occur- 
rences of  oil  as  seen  by  the  field  geologist. 

Organic  Theories. — By  organic  is  meant  any  chemical  process 
that  takes  place  by  assistance  of  living  organisms  such  as  bacteria, 
decomposing  vegetation,  or  animal  matter.  Some  scientists 
assert  that  oil  is  of  animal  origin,  others  that  it  comes  from  vege- 
table matter.  There  have  been  numerous  discussions  as  to  which 
is  the  more  likely  source.  A  compromise  view  asserts  that  petro- 
leum may  come  from  either  source  alone,  or  from  a  combination  of 
the  two.  It  has  been  claimed  that  oils  having  an  asphaltic  base 
are  derived  from  animal  matter  and  that  oils  with  a  paraffine 
base  are  derived  from  vegetable  matter.  Again  it  is  boldly  stated 
that  all  oils  are  derived  from  the  same  material  but  that  the  dif- 
ferences are  due  to  capillary  division,  differences  in  the  heat  and 
the  pressure  to  which  the  oil  has  been  subjected,  to  migration,  etc. 


PETROLEUM  3 

There  are  three  organic  theories  as  follows:  (1)  Animal  theo- 
ries; (2)  vegetal  theories;  and  (3)  combinations  of  animal  and 
vegetal  theories. 

In  discussing  the  following  theories  the  source  of  the  material 
is  alone  considered.  The  subject  of  the  derivation  of  petroleum 
from  organic  matter  is  treated  in  another  place. 

1.  ANIMAL  THEORIES. — One  theory  explains  that  oil  is  derived 
from  the  decomposition  of  the  bodies  of  marine  animals  such  as 
fish,  oysters,  scallops,  mollusks,  and  corals.     Some  bays    and 
coasts  literally  teem  with  marine  life  at  present,  and  it  is  assumed 
that  in  past  ages  marine  life  was  just  as  plentiful,  as  is  evidenced 
by  the  great  quantities  of  fossils  that  are  found  today.     The  death 
of  such  animals  and  their  subsequent  burial  in  the  marine  sedi- 
ments gave  material  sufficient  for  the  formation  of  oil. 

According  to  another  theory,  microscopic  organisms  called 
foramim'fera,  which  are  today  found  in  great  quantities  in  some 
places  along  sea  coasts,  furnished  the  material  for  oil.  These 
small  organisms  were  certainly  existent  in  great  quantities  in 
past  ages.  The  microscope  shows  that  beds  many  hundreds  of 
feet  thick  are  in  large  part  formed  of  these  organisms. 

2.  VEGETAL  THEORIES. — The  vegetable  theories  may  be  classi- 
fied under  the  following  heads :  (a)  The  sea-weed  theory;  (6)  the 
land-plant  theory;  (c)  the  diatom  theory;  and  (d)  the  coal  theory. 

(a)  The  sea-weed  theory  also  has  received  much  support. 
The  great  kelp  beds  that  line  some  sea  coasts,  notably  the  Pacific 
Ocean  and  the  Sargossa  sea  lend  strength  to  this  theory.  Cer- 
tainly there  is  material  enough  along  the  coasts  to  produce  a 
tremendous  quantity  of  oil  if  properly  distilled.  Supposing  that 
in  the  past  ages  as  large  quantities  of  material  existed,  and  were 
buried  in  sediments,  one  has  a  basis  for  a  strong  theory. 

(6)  The  land-plant  theory  is  based  upon  the  occurrence  of  great 
quantities  of  plants  found  in  land-locked  embayments,  in  swamps, 
and  in  low  marshes  and  lake  beds.  It  has  been  clearly  established 
that  coal  is  formed  from  plants  that  grew  in  great  swamps,  and  it 
is  assumed  that  similar  beds  of  material  under  different  condi- 
tions of  heat  and  pressure  gave  rise  to  petroleum  instead  of  to 


4  PRACTICAL  OIL  GEOLOGY 

coal.     Certainly  plants  have  all  the  constituents  necessary  to 
form  petroleum  so  that  such  a  theory  is  not  at  all  unreasonable. 

(c)  The  diatom  theory,  especially  advocated  by  California  geolo- 
gists, is  based  upon  the  study  of  the  microscopic  plants  that  are 
plentiful  in  many  parts  of  the  seas  and  oceans.     Many  carbona- 
ceous shales,  of  great  age  geologically,  contain  large  quantities 
of  these  minute  organisms.     The  presence  of  petroleum  in  these 
diatomaceous  shales  is  so  general  that  many  geologists  believe 
the  oil  originated  in  the  shales.     It  of  course  could  only  come 
from  the  microscopic  organisms. 

(d)  The  coal  theory  is  based  upon  the  fact  that  lignitic  and  bitu- 
minous coals  when  distilled  in  the  laboratory  yield  hydrocarbons 
similar  to  those  in  petroleum.     It  is  thought  that  similar  results 
are  obtained  in  nature  by  distilling  great  masses  of  coal  under 
proper  conditions  of  heat  and  of  pressure.     The  presence  of  coal 
in  many  oil  fields  lends  support  to  this  view  but  like  all  other 
theories  nothing  definite  has  been  established. 

COMBINATION  OF  ANIMAL  AND  VEGETAL  THEORIES. — In  some 
cases  one  finds  the  remains  of  animal  and  vegetable  material  in  the 
same  bed  or  stratum.  It  is  very  likely  where  such  has  been  the 
case  that  petroleum  has  been  derived  from  both  sources.  This 
view  at  least  reconciles  the  animal  and  the  vegetable  theories, 
and  in  no  way  conflicts  with  known  facts. 

Formation  of  Oil  from  Organic  Material. — The  formation  of 
petroleum  from  either  animal  or  vegetable  matter  is  considered 
to  be  as  follows: 

1.  The  organic  matter  is  first  laid  down  in  clays  and  sands  which 
have  been  deposited  under  water  along  sea  coasts,  in  swamps, 
bays,  or  in  lakes. 

2.  Other  beds  of  material  are  deposited  upon  those  carrying  the 
organic  matter,  until  a  thick  covering  is  formed. 

3.  The  water  and  the  overlying  sediments  protect  the  organic 
matter  from  rapid  destruction  by  oxidation,  and  especially  where 
the  water  is  salt,  it  acts  as  a  pickling  brine. 

4.  In  time  the  pressure  of  the  overlying  beds,  and  the  action  of 
heat,  which  is  supposedly  generated  by  the  pressure  of  the  over- 


PETROLEUM  5 

lying  sediments  or  by  the  action  of  plutonic  masses  of  rock  which 
have  been  intruded  into  the  sediments,  causes  a  distillation  of  the 
organic  matter  to  form  petroleum  products  which  are  later  ac- 
cumulated into  so-called  " pools"  or  fields  of  oil. 

Combination  of  Organic  and  Inorganic  Theories. — Several 
theories  combining  the  organic  and  the  inorganic  ideas  have  been 
offered  by  scientific  men.  The  principal  idea  of  all  these  theories 
is  that  gases  from  deep-lying  igneous  masses  pass  upward  through 
fissures  or  vents  in  the  earth's  surface,  and  coming  in  contact 
with  sediments  containing  organic  matter  form  hydrocarbon 
products.  There  is  little  positive  evidence  for  such  theories  ex- 
cept the  presence  of  volcanic  intrusions  in  a  few  oil  fields. 

So  far  as  known  the  organic  theories  seem  the  most  reasonable 
and  by  far  the  most  popular  with  scientific  men. 


THE  ACCUMULATION  OF  PETROLEUM  INTO  COMMERCIAL 

DEPOSITS 

The  origin  of  oil  is  one  problem,  its  accumulation  into  economic 
deposits  a-n  entirely  different  one.  More  is  known  about  the 
accumulation  of  oil  than  about  its  origin.  The  following  facts 
are  important  to  bear  in  mind  as  in  them  one  finds  the  key 
to  many  other  valuable  points  of  applied  geology  as  related  to 
petroleum. 

1.  Commercial  oil  and  gas  deposits  occur  in  the  higher  parts  of 
folds  or  wrinkles  of  the  earth's  surface  called  anticlines,  domes, 
monoclines,    etc. 

2.  Water  is  always  found  in  the  same  stratum  as  the  oil  but  in 
the  lower  part  of  the  fold. 

3.  All  commercial  deposits  so  far  have  occurred  in  sedimentary 
or  water-laid  deposits  such  as  sands,  sandstones,  conglomerates, 
shales,  and  limestones. 

4.  All  oil  and  gas  deposits,  so  far  as  known,  are  capped  or  covered 
by  practically  impervious  beds  of  shale,  sandstone,  or  limestone; 
also  such  deposits  are  underlaid  by  impervious  beds. 

The  discussion  of  these  points  is  given  under  the  following 


6 


PRACTICAL  OIL  GEOLOGY 


headings:  (1)  The  anticlinal  theory;  (2)  water  and  compression 
in  accumulation;  (3)  capillarity;  (4)  reservoirs  for  petroleum;  and 
(5)  impervious  beds  capping  and  underlying  the  oil  and  gas 
deposits. 

The  Anticlinal  Theory. — Anticline  is  the  name  given  to  the 
type  of  fold  that  is  arched  as  shown  in  Fig.  1.  Further  reference 
is  made  to  this  type  of  fold  in  Chapter  III.  As  the  anticline  is 
the  most  common  form  of  fold  found  in  the  oil  fields  the  theory  of 
oil  accumulations  in  folds  was  given  the  name  anticlinal  theory, 
although  several  other  types  of  folds  also  act  as  oil  reservoirs. 

Originally  the  sedimentary  strata  were  laid  down  along  sea 
coasts,  in  swamps,  lakes,  etc.,  as  flat  or  horizontal  beds.  Suppose 
a  large  flat  bed  of  sand,  sandstone,  conglomerate,  shale,  or  lime- 
stone, carrying  oil,  gas,  and  water  throughout  it,  to  lie  buried 


FIG.  1. — Illustration  of  ideal  anticlinal  conditions.1 


under  a  mass  of  sediments  which  are  impervious  or  nearly  so; 
suppose  also  that  this  stratum  is  underlaid  by  impervious  beds. 
Conditions  such  as  assumed  above  are  common  along  many  sea 
coasts,  in  bays,  and  in  gulf  regions.  In  such  a  flat  stratum  it  is 
found  that  the  gas  and  oil  will  rest  upon  the  top  of  the  water  due 
to  the  differences  in  specific  gravity,  gas  and  oil  being  lighter 
than  water.  In  drilling  through  such  a  flat  stratum  the  drill  will 
encounter  first  a  layer  of  gas,  then  a  layer  of  oil  and  last  a  layer  of 
water.  However,  in  such  flat  strata  oil  will  not  be  found  in  pay- 
ing quantities.  To  obtain  commercial  production  another  con- 
dition is  essential.  There  must  be  a  sufficient  quantity  of  oil  and 
gas  to  pay  for  its  extraction,  and  such  accumulations  are  found 
1  For  symbols  used  in  this  book,  see  Fig.  53,  p.  89. 


PETROLEUM  7 

where  the  flat  strata  have  been  thrown  into  arches  or  folds  like 
that  shown  in  Fig.  1.  As  will  be  noticed  in  studying  Fig.  1,  the 
same  stratified  or  layer-like  relations  of  the  gas,  oil  and  water 
occur  as  they  would  in  perfectly  horizontal  beds.  At  the  top  of 
the  arch  or  anticline  is  gas,  below  the  gas  is  oil,  and  at  the  base  of 
the  fold  is  the  water.  This  is  the  theoretical  condition  and  is 
closely  approximated  in  nature.  Under  certain  conditions,  how- 
ever, oil  and  gas  both  occur  at  the  top  of  the  arch,  and  under  most 
conditions  there  is  more  or  less  gas  in  the  oil  on  the  flanks  or  sides  of 
the  structure.  The  above  practically  covers  the  anticlinal  theory. 

Water  and  Compression  in  Oil  Accumulations. — In  discussing 
the  anticlinal  theory  one  notices  that  the  water  was  not  assumed 
to  be  under  pressure,  but  that  the  oil  merely  floated  upon  the  top 
of  the  water  due  to  differences  in  specific  gravities,  much  the 
same  as  a  cork  floats  upon  water.  The  water  occupies  the  lower 
parts  of  the  folds  because  of  its  tendency  to  seek  its  level,  a  well- 
recognized  truism.  Normally  under  such  conditions  water  occurs 
in  the  basins  or  depressions  called  synclines,  the  opposite  of 
anticlines. 

Suppose2  however,  that  there  are  several  parallel  anticlines  or 
several  domes  on  one  anticline  (see  Fig.  42,  Chapter  V) .  The  lower 
arches  are  under  the  hydraulic  pressure  of  the  oil  and  the 
water  in  the  higher  arches.  In  such  a  case  water  will  occupy 
the  lower  anticlines  wherever  the  hydraulic  pressure  is  great 
enough  to  drive  the  oil  higher  on  the  slope.  Note  that  the 
lowest  anticline  carries  no  oil.  The  oil  at  the  top  of  the  struc- 
ture is  not  under  pressure  to  any  appreciable  extent.  In  the 
above  theory  the  water  is  not  static  or  stationary,  but  is  meteoric 
or  rain  water,  which  enters  the  outcropping  sands,  and  works  its 
way  downward  into  the  oil  stratum.  Also  the  movement  of  the 
water  may  be  due  to  the  rising  or  falling  of  the  whole  land  mass. 
Thus  a  rising  region  would  result  in  lowering  the  water  level, 
and  a  sinking  region  would  result  in  raising  the  water  level. 

Where  faults  or  where  unconformities  occur  (see  Chapters  II, 
III  and  IV),  water  by  driving  oil  from  the  lower  sands  will  force  it 
to  enter  strata  above.  Such  traveling  of  oil  is  called  migration 


8  PRACTICAL  OIL  GEOLOGY 

which,  however,  is  not  dependent  alone  upon  water  pressures. 
Other  factors  such  as  compression  discussed  below,  and  capillarity 
discussed  under  specific  gravity,  also  assist  in  migration  of  the 
oil  from  one  formation  to  another.  The  part  that  compression 
plays  in  oil  accumulation  would  seem  to  merit  more  careful  atten- 
tion than  has  heretofore  been  accorded  the  subject.  Many  shales 
are  saturated  with  petroleum.  If  these  shales  were  under  suf- 
ficiently great  pressure  the  oil  would  be  forced  from  them. 
Tremendous  pressures  are  set  up  by  earth  folding.  Such  being 
the  case  it  is  not  at  all  unlikely  that  the  shales  in  the  tops  and  at 
the  bottoms  of  the  folds  may  be  so  compressed  that  part  of  their 
petroleum  content  would  be  squeezed  out  of  the  shale  body.  If 
porous  sands  or  sandstones  are  above  or  below  the  shale,  the  oil 
would  be  forced  to  migrate  into  the  porous  beds.  Sand  and  sand- 
stones are  generally  more  porous  than  shales  and  they  form  better 
reservoirs  than  do  the  shales. 

One  of  the  important  features  of  accumulation  is  the  porosity 
of  the  beds  at  the  top  of  the  fold.  If  the  beds  at  the  top  of  the 
fold  are  not  porous,  due  either  to  their  being  hard  compact  shales 
or  sandstones  little  or  no  accumulation  will  take  place. 

CAPILLARITY  (After  WASHBURNE) 
"Extract  from  Washburne  Letter" 

"The  only  matter  which  I  consider  as  practically  proven  by  my 
study  of  the  geophysics  of  petroleum  is  the  control  of  capillarity  upon 
the  distribution  of  gas,  oil,  and  water  within  the  rocks.  Since  water  has 
about  50  per  cent,  higher  surface  tension  than  oil,  it  tends  to  be  drawn 
into  the  finest  capillaries  with  half  again  as  much  force  as  that  drawing 
oil  into  the  fine  openings.  Every  slight  movement  of  the  various  fluids 
in  the  rocks  tends  to  move  water  from  sandstone  into  shale  with  greater 
ease  than  the  reverse  tendency  from  shale  into  sandstone.  Likewise, 
it  tends  to  move  gas  from  shale  into  sandstone  with  greater  ease  than 
from  sandstone  into  shale.  The  net  result  of  this  tendency,  which  has 
operated  continuously  since  the  formation  of  the  strata,  is  to  cause  the 
concentration  of  gas  and  oil  into  coarse  spaces  of  rocks,  that  is,  in  fis- 
sures or  sandstone  layers  in  shale,  in  conglomeratic  or  other  coarse  layers 
in  sandstone,  etc.,  leaving  the  water  within  the  shale.  I  believe  that 


PETROLEUM     '  9 

this  exchange  of  gas  and  water  between  sandstone  layers  and  shale  is  one 
of  the  causes  of  the  apparent  dryness  of  the  deep  sands  of  the  Appala- 
chian and  Mid-continent  oil  fields.  These  sands  were  originally  full 
of  water,  but  in  the  course  of  geologic  time  this  water  has  been  exchanged 
for  nitrogen,  methane,  and  carbon-dioxid  of  the  adjacent  shales  through 
the  operation  of  capillary  forces." 

Reservoirs  for  Petroleum. — As  shown  above,  the  higher  parts 
of  folds  act  as  great  collecting  reservoirs  for  oil.  A  study  of  the 
strata  making  up  such  reservoirs  will  prove  of  value.  The  best 
reservoirs  for  oil  are  coarse  sands,  conglomerates,  and  porous 
dolomitic  limestones.  (See  Chapter  III  for  definitions.)  Sand- 
stones and  shales  often  carry  oil,  but  they  are  not  the  most  favor- 
able for  reservoirs,  as  in  most  sandstones  the  cementing  material 
binding  the  sand  grains  together  fills  the  pores  so  that  the  rock 
can  hold  only  a  small  quantity  of  fluid.  Shales  also  have  very 
fine  pores  and  hold  only  small  quantities  of  oil,  except  where  the 
shales  have  been  broken  into  fragments  due  to  intense  crushing 
as  is  the  case  with  some  of  the  California  shales,  especially  the 
silicified  Monterey  shales  at  Santa  Maria. 

yThe  percentage  of  voids  in  the  various  kinds  of  strata  varies 
considerably.  Sands  may  contain  from  15  to  25  per  cent,  voids; 
sandstones,  5  to  15  per  cent,  voids;  conglomerates  may  contain  as 
high  as  30  per  cent,  voids;  shales,  from  2  to  10  per  cent.,  and  some 
dolomitic  limestones  are  reported  to  contain  as  high  as  35  per  cent, 
voids.  The  factors  above  are  so  variable  that  one  must  not  take 
the  material  in  one  field  to  be  a  criterion  or  measure  of  material 
in  other  fields.  The  following  discussions  on  the  quantity  of  oil 
in  the  sands,  saturation,  and  drainage  areas  are  all  interesting  and 
pertinent  to  the  above  discussion  on  voids. 

QUANTITY  OF  OIL  IN  SANDS. — Many  people  think  of  lakes  of 
oil  lying  underground.  Such  is  not  the  case,  by  any  means.  It 
is  entirely  unnecessary  to  call  such  a  theory  into  use  to  explain  the 
oil  reservoirs.  The  small  voids  in  the  sands  afford  plenty  of  space 
in  which  oil  may  accumulate.  Some  sands  contain  20  per  cent, 
voids.  If  these  voids  were  full  of  oil,  each  100  cu.  ft.  of  sand  would 
contain  20  cu.  ft.  of  oil.  A  bed  100  ft.  thick  and  covering  an  acre 


10  PRACTICAL  OIL  GEOLOGY 

of  land  would  then  contain  the  following  number  of  barrels  of  oil 
(42  gallons  per  barrel — 7.5  gallons  per  cu.  ft.) : 

43,560  X  7.5  X  20 

-  =  loOjOoo  barrels. 

SATURATION  OF  OIL  SANDS. — By  oil  saturation  is  meant  the 
percentage  of  oil  present  by  volume  in  a  cubic  foot  of  oil  sand. 
If  the  voids  are  20  per  cent.,  and  the  sand  is  filled  with  petro- 
leum, then  the  saturation  is  20  per  cent . l  However,  this  method  is 
only  a  rough  approximation,  as  the  amount  of  oil  that  a  formation 
holds  depends  not  only  upon  the  porosity  but  upon  the  tempera- 
tures of  the  earth,  the  hydrostatic  and  the  rock  pressures.  Only 
when  all  these  factors  are  known  can  one  obtain  an  accurate  idea 
of  the  saturation. 

The  U.  S.  Government  in  its  estimates  takes  10  per  cent,  as 
the  saturation.  Other  estimates  allow  1  gallon  of  oil  to  each 
cubic  foot  of  sand  (13.3  per  cent.),  or  approximately  1000  barrels 
per  acre-foot.  By  10  per  cent,  saturation  is  not,  however,  meant 
that  10  per  cent,  of  the  oil  is  recoverable.  The  amount  of  recov- 
erable oil  may  only  be  50  per  cent,  of  the  saturation  or  it  may  be  as 
high  as  75  per  cent,  varying  with  the  porosity  of  the  sand,  gas 
pressure,  etc.  This  will  be  discussed  in  Chapter  VII. 

As  one  can  readily  appreciate,  it  is  entirely  unnecessary  to 
assume  lakes  to  account  for  the  oil  below  the  ground.  In  some 
places  where  the  sands  are  200  to  300  ft.  thick  enormous  quanti- 
ties of  petroleum  are  found;  in  others  where  the  beds  are  but  10  ft. 
thick  a  correspondingly  less  quantity  of  oil  is  found,  though  this 
holds  true  only  under  certain  conditions. 

A  sand  100  ft.  thick  fully  saturated  with  oil  will  produce  more 

011  than  one  only  10  ft.  thick,  provided,  of  course,  the  size  of  sand 
grain  and  per  cent,  of  saturation  are  the  same,  and  the  gas  pres- 
sure and  dips  are  constant  in  both  cases.     Some  thin  sands  have 
been  very  rich  producers  where  coarse  grained,  producing  for  a 
time  much  larger  quantities  of  oil  than  thicker  beds  which  were 
finer  grained. 

1Some  scientists  call  the  complete  saturation  100%.  If  but  10%  of  the 
voids  of  the  sand  were  filled  the  saturation  would  be  50%. 


PETROLEUM  11 

Porous  beds  under  heavy  gas  pressures  give  up  their  oil  very 
rapidly.  A  thin  bed  of  sand  or  limestone  which  is  of  coarse  tex- 
ture may  then  be  expected  to  have  a  short  life  compared  with  thick 
beds  of  finer  grained  material. 

The  longevity  of  a  well  does  not  determine  its  productivity, 
as  one  well  may  produce  in  one  month  as  much  oil  as  others 
could  produce  in  a  lifetime  of  15  or  20  years.  The  great 
gushers  like  the  Lucas  gusher  of  Spindle  Top,  Texas,  and  the 
Lakeview  gusher  of  California  are  examples  of  great  productivity. 
Such  wells  produced  phenomenally  for  comparatively  short 
periods  of  time,  but  soon  exhausted  themselves. 

DRAINAGE  AREAS. — It  has  been  explained  above  how  oil 
accumulates  and  some  idea  was  given  as  to  the  quantities  in  oil 
strata.  In  conclusion  it  is  well  to  touch  on  the  size  of  the  areas 
from  which  the  oil  collects.  Drainage  area  is  rather  a  misnomer 
as  oil  does  not  generally  travel  downward,  but  is  displaced  by 
water  and  thus  rests  upon  it.  However,  the  term  expresses  the 
same  idea  as  drainage  and  will  be  used  here  to  mean  that  area 
from  which  the  oil  has  collected. 

NaturaDy  the  size  of  folds  varies  greatly.  Such  being  the  case 
it  follows  that  the  larger  the  fold,  the  more  oil  it  should  hold, 
other  conditions  being  equal.  The  measurement  of  a  fold  is 
not  a  difficult  matter  once  the  basins  or  depressions  are  deter- 
mined. If  an  anticline  is  25  miles  long  and  10  miles  wide,  the 
drainage  area  is  not  25  X  10,  or  250  square  miles,  but  will  be 
greater,  depending  upon  the  shape  of  the  fold.  A  fold  shaped 
like  that  in  Fig.  10a  would  not  cover  as  large  an  area  as  the  fold 
in  106  or  in  lOc  (all  shown  in  Chapter  III),  if  the  folds  were  flat- 
tened out.  A  few  folds  measured  by  the  writer  had  relative  areas 
of  300,  80,  5,  and  J£  square  miles.  The  large  folds  give  large 
acreages  of  available  oil  land,  while  the  smallest  fold  would  not 
pay  to  prospect.  It  must  not  be  thought  that  all  of  the  area 
carries  oil.  The  proportion  of  oil  land  in  average  oil-field  drain- 
age areas  varies  from  1  to  20  per  cent,  of  the  total  drainage 
area;  however,  5  per  cent,  would  be  a  large  proportion  for  most 
oil  fields.  The  areas  given  above  on  a  5  per  cent,  basis  would 


12  PRACTICAL  OIL  GEOLOGY 

furnish  available  acreages  of  9600,  2560,  128  and  4  acres  re- 
spectively. One  cannot,  however,  figure  on  such  an  average  for 
the  true  areas  were  approximately  5000,  500  and  100  acres  re- 
spectively. The  smaller  area  was  not  tested.  From  these  few 
notes  one  can  appreciate  that  each  field  is  a  problem  by  itself. 

Impervious  Beds,  Capping  and  Underlying  the  Oil  Strata. — 
Oil  strata  are  overlaid  or  capped,  and  underlaid  by  strata  prac- 
tically impervious  to  oil.  These  beds  may  consist  of  compact 
shales,  hard,  closely  cemented  sandstones,  and  compact  limestones. 
In  all  cases  the  overlying  strata  must  be  tight  enough  to  effec- 
tually seal  in  the  oil.  Where  the  strata  have  been  eroded  leaving 
exposed  sands,  the  more  volatile  oils  will  escape  unless  asphaltic 
or  paraffine  deposits  coat  the  faces  of  the  strata,  and  act  as  seals 
to  keep  in  the  remaining  oil. 

However,  as  explained  under  "Effects  of  Migration,"  there 
are  conditions  where  oil  may  work  its  way  upward  through  the 
overlying  beds.  Also  were  it  not  for  the  compact,  overlying 
beds,  water  would  work  its  way  downward  from  the  surface  of 
the  earth  and  flood  the  oil  strata. 

It  is  just  as  essential  to  have  a  compact  underlying  bed  of 
material  to  retain  the  oil  as  it  is  to  have  a  capping.  If  it  were 
not  for  such  beds  the  oil  would  gradually  escape  downward  from 
the  stratum  in  which  it  originally  occurred.  It  seems  highly 
probable  that  such  has  been  the  case  in  some  instances.  How- 
ever, water  in  the  lower  beds  would  stop  migration,  for  petroleum 
will  not  escape  through  a  rock  saturated  with  water.  This 
applies  whether  or  not  the  water  stratum  is  above  or  below  the 
oil  stratum.  In  such  saturated  rocks  the  water  is  held  by 
friction  and  capillary  attraction  to  the  small  grains  of  sediment 
and  the  oil  has  not  sufficient  pressure  to  overcome  these  factors. 
If  for  any  reason  the  rocks  below  lose  their  water  the  oil  will  under 
some  conditions  travel  downward. 


PETROLEUM  13 

DYNAMIC  HEAT  GENERATED  BY  INTENSE  FOLDING  AND  ITS 
EFFECT  ON  OIL  FIELDS 

A  theory  advanced  by  David  White  seems  to  be  applicable  in 
the  oil-fields  of  the  Appalachian  and  Mid-Continent  regions. 

Briefly  stated,  the  theory  is  that  as  one  approaches  the  lines 
or  centers  of  intense  folding  the  heat  generated  by  the  intense 
folding  has  been  sufficient  to  volatilize  the  hydrocarbons  in  the 
oil  and  change  them  to  a  more  stable  gaseous  form. 

Metamorphism  has  affected  the  coals  to  the  extent  that  the 
closer  the  coal  is  found  to  the  centers  of  lines  of  uplift  the  more 
anthracitic  the  coal  becomes,  i.e.,  contains  more  fixed  carbon  and 
less  volatile  water. 

This  is  particularly  the  case  in  the  Appalachian  fields  and 
holds  without  any  question  in  the  southeastern  Oklahoma  and 
Arkansas  gas  fields. 

Vast  quantities  of  gas  are  formed  with  but  small  traces  of  oil. 
The  theory  is  certainly  entitled  to  serious  consideration,  and  in 
Oklahoma  and  Arkansas  it  has  formed  an  excellent  working  guide 
to  petroleum  geologists. 


CHAPTER  II 

PETROLEUM— PHYSICAL  AND  CHEMICAL  PROPERTIES 
DIFFERENCES  IN  SPECIFIC  GRAVITY  OF  VARIOUS  OILS 

Specific  Gravity. — The  specific  gravity  of  any  fluid  is  the  rela- 
tion the  fluid  bears  by  weight  to  the  same  volume  of  water. 
Water  has  a  specific  gravity  of  1.  As  petroleum  is  lighter  than 
water,  its  specific  gravity  is  expressed  by  decimals  less  than 
unity.  Gravity  is  also  expressed  in  degrees  Baume,  a  method 
employed  by  a  French  chemist  to  measure  the  comparative  weight 
of  fluids.  The  greater  the  degrees  Baume  the  lighter  the 
fluid.  The  relation  between  the  two  methods  is  shown  and 
explained  in  Table  I. 

The  instruments  used  are  a  hydrometer  and  a  standard  ther- 
mometer. The  hydrometer,  which  is  a  glass  column  marked  with 
graduations  from  10  to  100,  was  invented  by  Antoine  Baume, 
a  French  chemist,  and  the  scale  on  the  instrument  has  always 
borne  his  name.  The  hydrometer  when  placed  in  a  jar  or  a  bottle 
of  oil  sinks  to  the  point  on  the  scale  which  indicates  the  gravity 
in  degrees  Baume.  The  basis  of  temperature  for  testing  oil  is 
60°  F.  and  for  oil  at  a  greater  or  less  temperature,  variations 
must  be  calculated.  Hydrometers  are  usually  provided  with 
a  special  scale  for  figuring  temperature  variations.  The  specific 
gravity  is  found  by  dividing  140  by  130  plus  the  Baume*  de- 
grees, for  example:  if  the  hydrometer  registers  30°,  this  added 
to  130  equals  160,  which  divided  into  140  shows  specific 
gravity  0.875°. 

14 


PETROLEUM 


15 


Following  is  a  table  showing  Baume  degrees,  specific  gravity, 
and  weight  per  gallon  of  oil. 

TABLE  I. — SPECIFIC  GRAVITY  OF  CRUDE  OIL  AND  METHOD  OF  FINDING  IT 


Degrees 
Beaum6 

$  0  >> 

Ml 

I'S 

g| 

Degrees 
specific 
gravity 

£  a& 

C  £ 

S  c  >> 

M  d  t* 
0><1)  c« 

olfe 

|+9         00 

I  •£  ^  a~o 

10 

1  .  0000 

8.33 

32 

0.8641 

7.20 

54 

0.7608 

6.34 

11 

.9929 

8.27 

33 

.8588 

7.15 

55 

.7567 

6.30 

12 

.9859 

8.21 

34 

.8536 

7.11 

56 

.7526 

6.27 

13 

.9790 

8.16 

35 

.8484 

7.07 

57 

.7486 

6.24 

14 

.9722 

8.10 

36 

.8433 

7.03 

58 

.7446 

6.20 

15 

.9655 

8.04 

37 

.8383 

6.98 

59 

.7407 

6.17 

16 

.9589 

7.99 

38 

.8333 

6.94 

60 

.7368 

6.14 

17 

.9523 

7.93 

39 

.8284 

6.90 

61 

.7329 

6.11 

.18 

.9459 

7.88 

40 

.8235 

6.86 

62 

.7290 

6.07 

19 

.9395 

7.83 

41 

.8187 

6.82 

63 

.7253 

6.04 

20 

.9333 

7.78 

42 

.8139 

6.78 

64 

.7216 

6.01 

21 

.9271 

7.72 

43 

.8092 

6.74 

65 

.7179 

5.98 

22 

.9210 

7.67 

44 

.8045 

6.70 

66 

.7142 

5.95 

23 

.9150 

7.62 

45 

.8000 

6.66 

67 

.7106 

5.92 

24 

.9090 

7.57 

46 

.7954 

6.63 

68 

.7070 

5.89 

25 

.9032 

7.53 

47 

.7909 

6.59 

69 

.7035 

5.86 

26 

.8974 

7.48 

48 

.7865 

6.55 

70 

.7000 

5.83 

27 

.8917 

7.43 

49 

.7821 

6.52 

75 

.6829 

5.69 

28 

.8860 

7.38  ' 

50 

.7777 

6.48 

80 

.6666 

5.55 

29 

.8805 

7.34 

51 

.7734 

6.44 

85 

.6511 

5.42 

30 

.8750 

7.29 

52 

.7692 

6.41 

90 

.6363 

5.30 

31 

.8695 

7.24 

53 

.7650 

6.37 

95 

.6222 

5.18 

To  account  for  the  differences  in  specific  gravity  of  petroleum 
a  number  of  theories  have  been  presented,  as  shown  below.  It 
is  especially  important  to  note  the  economic  side  of  the  ques- 
tion as  classified  under  that  head. 

EFFECTS  OF  MIGRATION. — Oil  is  not  generally  indigenous  to 
the  formation  in  which  it  is  found,  but  has  migrated  from  other 
formations.  The  migration,  or  travel,  of  petroleum  from  one 
formation  to  another  undoubtedly  affects  its  specific  gravity. 

Petroleum  is  a  mixture  of  hydrocarbons,  each  having  different 


16  PRACTICAL  OIL  GEOLOGY 

specific  gravities.  If  petroleum  occurs  in  a  sand,  portions  of 
it  may  work  upward  or  downward  through  the  capping  above  or 
the  bottom  formation  underlying  the  sand.  If  the  cappings 
or  bottoms  are  very  fine  grained,  only  a  very  small  proportion 
of  the  hydrocarbons  will  escape.  If  shale  overlies  the  sand  the 
lighter  part  of  the  hydrocarbons  will  work  its  way  through 
the  shale  to  the  strata  above.  The  heavy  constituents  will  be 
left  in  the  sand  below,  some  lighter  constituents  will  be  found 
in  the  shale,  and  still  lighter  constituents  in  the  formations 
above.  If  the  migration  is  downward,  the  oils  below  will  be 
lighter  than  those  above. 

A  very  thick  oil  will  not  penetrate  fine-grained  clay  nor  shale, 
so  there  is  a  limit  to  the  amount  of  petroleum  that  will  escape 
from  the  original  formation. 

Where  there  are  faults  or  breaks  in  the  formations,  allow- 
ing the  escape  of  hydrocarbons  from  deep-lying  carbonaceous 
formations  to  overlying  strata,  the  lighter  constituents  may  es- 
cape from  the  lower  beds  and  be  found  above.  Water  as- 
cending along  these  fault  planes  may  carry  the  petroleum  with 
it.  Necessarily  the  petroleum  that  is  mixed  with  water  will  be 
heavier  than  the  original  petroleum. 

ECONOMIC  ASPECT  OF  SPECIFIC  GRAVITY. — As  it  is  known  that 
petroleum  in  strata  of  different  ages  varies,  it  is  of  course  ad- 
visable to  know  the  quality  of  the  oil  desired  and  bore  for  that 
stratum.  For  that  reason,  if  no  other,  it  is  often  important  to 
know  the  ages  of  the  formations  through  which  the  drill  is  to 
penetrate.  It  is  generally  true  that  the  high-gravity  oils  occur 
in  formations  that  are  much  younger  than  those  containing 
the  low^gravity  oils,  although  there  are  exceptions  to  this  rule. 

The  position  of  a  well  on  the  fold  is  most  important  as  regards 
the  gravity  of  the  oil  to  be  encountered.  In  a  closed  structure 
such  as  that  in  Fig.  1,  the  lightest  oil  will  occur  near  the  gas 
line  and  the  heavier  oil  near  the  water  line,  for  petroleum  is  not 

xOn  the  Baumc  scale  the  gravity  decreases  with  the  number  of  degrees. 
High  gravities  would  be  smaller  numbers  than  low  gravities  (see  Table  1,  p.  13). 


PETROLEUM  17 

a  homogeneous  fluid  but  a  mixture  of  a  number  of  hydrocarbons 
which  separate  in  a  vessel  according  to  their  specific  gravities. 

Where,  however,  there  is  an  open  structure  like  that  in  Fig.  4, 
Chapter  II,  the  oil  near  the  outcrop  will  be  heavy,  due  to  the 
escape  of  the  volatile  constituents;  the  oil  a  little  further  down 
the  dip  will  be  lighter,  and  then  heavier  oil  will  be  encountered 
near  the  water  line. 

Where  faulting  exposes  the  beds  or  allows  the  escape  of  oil 
along  the  fault  plane,  heavy  oil  may  be  expected  in  proximity  to 
the  fault. 

CHEMICAL  COMPOSITION  OF  PETROLEUM1 

Natural  gas,  petroleum,  bitumen,  and  asphaltum  are  all 
essentially  compounds  of  carbon  and  hydrogen,  or,  more  precisely, 
mixtures  of  such  compounds  in  bewildering  variety.  They  con- 
tain, moreover,  many  impurities — sulphur  compounds,  oxidized 
and  nitrogenous  substances,  etc. — whose  exact  nature  is  not  always 
clearly  defined.  The  proximate  analysis  of  a  petroleum  or  bitumen 
consists  in  separating  its  components  from  one  another,  and  in 
their  identification  as  compounds  of  definite  constitution. 

All  of  the  hydrocarbons  fall  primarily  into  a  number  of  regular 
series,  to  each  of  which  a  generalized  formula  may  be  assigned, 
in  accordance  with  the  following  scheme: 

1.  CnH2ll+2  6.  CnH2n_8 

2.  CnH2n  7.  CnH2n_io 

3.  CnH2n_2  8.  CnH2n_12 

4.  CnH2n_4  -    

5.  CnH2n_6  18.  CnH2n_32 
Members  of  the  first  eight  series  have  been  discovered  in  petro- 
leum.    These  expressions,  however,  have  only  a  preliminary  value, 
although  they  are  often  used  in  the  classification  of  petroleum. 
Each  one  represents  a  group  of  series — homologous,  isomeric,  or 
polymeric,  as  the  case  may  be — and  for  precise  work  these  must 
be  taken  separately.     The  first  formula,  for  example,  represents 
what  are  known  as  the  paraffine  hydrocarbons,  which  begin  with 

1  After  Clark,  Data  of  Geo.  Chemistry. 

2 


18  PRACTICAL  OIL  GEOLOGY 

marsh  gas  or  methane,  CH4,  and  range  at  least  as  high  as  the 
compound  C35H72.  Even  these  are  again  subdivided  into  a 
number  of  isomeric  series — the  primary,  secondary,  and  tertiary 
paraffines — which,  with  equal-percentage  composition,  differ  in 
physical  properties,  by  virtue  of  differences  of  atomic  arrangement 
within  the  molecules.  Each  member  of  the  series  differs  from  the 
preceding  member  by  the  addition  to  the  group  CH2,  and  also 
by  the  physical  characteristics  of  greater  condensation.  Methane, 
CH4,  for  example,  is  gaseous;  the  middle  members  of  the  series 
are  liquids,  with  regularly  increasing  boiling  points;  the  higher 
members  are  solids,  like  ordinary  paraffine.  These  hydrocarbons 
are  especially  characteristic  of  the  Pennsylvania  petroleum,  from 
which  the  following  members  of  the  series  have  been  separated. 

To  the  list  in  Table  II,  the  isomeric  secondary  paraffines,  iso- 
butane,  isopentane,  isohexane,  and  isooctane  must  be  added,  and 
even  then  the  list  is  probably  not  complete.  For  instance,  the 
solid  paramnes  C27H56  and  C30H62  have  been  found  in  petroleum. 

Natural  gas  consists  almost  entirely  of  paramnes,  mainly  of 
methane,  with  quite  subordinate  impurities.  In  six  samples 
from  West  Virginia,  analyzed  by  C.  D.  Howard,  the  total  paramnes 
varied  between  94.13  and  95.73  per  cent.;  methane,  from  79.95 
to  86.48  per  cent,  and  ethane,  from  7.65  to  15.09.  The  fol- 
lowing analyses  from  other  sources  may  be  cited  more  in  detail. 
(See  Table  III. ) 

The  analyses  of  Pennsylvania  gases  by  S.  P.  Sad  tier  gave  some- 
what different  results.  In  gas  from  four  different  wells  he  found 
the  following:  CH4,  60.27  to  89.65  per  cent.;  C2H6,  4.39  to  18.39; 
and  H2,  4.79  to  22.50.  The  high  figures  for  hydrogen  are  unusual 
and  suggest  a  resemblance  to  coal  gas.  In  all  cases,  however, 
methane  is  the  preponderating  constituent,  the  characteristic 
hydrocarbon  of  natural  gas.  In  the  natural  gas  of  Point  Abino, 
Canada,  F.  C.  Phillips  found  96.57  per  cent,  of  paramnes  and 
0.74  of  H2S. 

Hydrocarbons  of  the  form  CnH2n  are,  as  constituents  of  pe- 
troleum, of  equal  importance  to  the  paraffines.  These  again  fall 
into  several  independent  series,  which  vary  in  physical  properties 
and  in  their  chemical  relations,  but  are  identical  in  percentage 


PETROLEUM 


19 


TABLE    II. — PARAFFINES  FROM  PENNSYLVANIA  PETROLEUM 


Name 

Formula 

Melting 
point 

Boiling 
point 

1.  Gaseous: 
Methane 

CH4 

°c. 

-186 

°c. 

—  164 

Ethane  

-172.1 

-  84.1 

Propane   .               .      .             

C3H8.. 

-  37 

Butane 

C4H10. 

' 

-f-     1 

2.  Liquid: 
Pentane 

37 

Hexane  

69 

Heptane  

98 

Octane 

125 

Nonane  
Decane 

CgH^O'  -  - 

-  51 
-  31 

150 
173 

Endecane  
Dodecane  

CnH.24-  • 

-  26 
-   12 

195 
214 

Tridecane 

Tetradecane  
Pentadecane  . 

CuHao.  . 

+     4 

252 

Hexadecane  

18 

3.  Solid:       -„ 
Octodecane 

Eicosane  

37 

Tricosane 

48 

Tetracosane  
Pentacosane  

C24H60.  . 

50-51 
53-54 

Hexacosane  

55-56 

Octocosane  
Nonocosane 

C2sH58. 

C    H 

60 
62-63 

Hentria  contane 

r  H 

66 

Dotriacontane  
Tetratriacontane 

C32Hc6.  • 

C    H 

67-68 
71-72 

::::::::: 

Pentatriacontane1  

CssH^.  . 

76 

1  For  a  description  of  these  higher,  solid  paraffines,  see  C.  F.  Mabery,  Am. 
Chem.  Jour.,  vol.  33,  p.  251,  1905.  The  literature  of  these  substances  is  so 
voluminous  that  I  cannot  attempt  to  give  exhaustive  references.  C.  Hell 
and  C.  Hagele  (Ber  Deutsch.  chem.  Gesell.,  vol.  22,  p.  504,  1889)  have 
described  an  artificial  hydrocarbon,  C6oHi22. 


20 


PRACTICAL  OIL  GEOLOGY 


TABLE  III. — ANALYSES  OF  NATURAL  GAS 


A 

B 

C 

D 

* 

E 

F 

CH4...   

93.36 

97.63 

Paraffines1  
C2H4,  etc 

96.36 

98.90 

87.27 

93.56 

0  28 

0  22 

CO 

0  53 

1  32 

C02  
H2 

3.64 
none 

0.40 

none 

0.41 
none 

0.14 
none 

0.25 
1  76 

0.22 
none 

N2  

none 

.70 

12.32 

6.30 

3.28 

0.60 

H2S  

none 

none 

none 

none 

0.18 

O2                        .      . 

none 

none 

none 

none 

0  29 

trace 

100.00 

100.00 

100.00 

100.00 

99.93 

100.00 

A.  From  Creighton,  Pennsylvania. 

B.  From  Pittsburg,  Pennsylvania. 

C.  From  Baden,  Pennsylvania. 

D.  From   Vancouver,    British   Columbia.     Analyses  A  to   D  by   F.    C. 
Phillips,  Am.  Chem.  Jour.,  vol.  16,  p.  406,  1894.     Selected  from  a  table  of 
seventeen  analyses  to  show  extreme  variations. 

E.  Mean  of  four  gases  from  Indiana  and  three  from  Ohio,  analyzed  by  C. 
C.  Howard  for  the  United  States  Geological  Survey.    Cited  by  W.  J.  McGee, 
Eleventh  Ann.  Kept.,  U.  S.  Geol.  Survey,  pt.  1,  p.  592,  1891. 

F.  From  Oswatamie,  Kansas.     From  a  table  of  seven  analyses  by  E.  H. 
S.  Bailey,  Kansas  Univ.  Quart.,  vol.  4,  p.  1,  1895.     According  to  H.  P.  Cady 
and  D.  F.  McFarland  (Trans.  Kansas  Acad.  Sci.,  vol.  20,  p.  80,  1907),  the 
natural  gas  of  Kansas  contains  helium.     It  was  found  in  forty-four  samples, 
in  amounts  from  0.01  to  nearly  2  per  cent. 

composition.  One  series,  the  olefines,  is  parallel  to  the  paraffine 
series,  and  the  following  members  of  it  are  said  to  have  been  iso- 
lated from  petroleum. 

Table  (IV)  is  probably  exact  in  an  empirical  sense,  but  not 
so  constitutionally.  Hydrocarbons  of  the  indicated  composition 
have  undoubtedly  been  found,  and  some  of  them  are  certainly 
olefines.  According  to  C.  F.  Mabery,  however,  the  true  olefines, 
or  "open-chain"  series,  are  present  in  petroleum  at  most  in  very 
small  amounts.  In  Canadian  petroleum  Mabery  and  W.  0. 
Quayle  identified  hexylene,  heptylene,  octylene  and  nonylene. 

largely  CH4,  with  more  or  less  ethane.     CO  not  found  by  Phillips. 


PETROLEUM 


21 


In  other  cases,  and  notably  in  the  Russian  petroleums,  the  com- 
pounds CnH2n  are  not  olefines,  but  cyclic  hydrocarbons  of  the 
polymethylene  series,  which  were  originally  called  naphtenes. 
They  were  at  first  supposed  to  be  derivatives  of  the  benzene 
series,  and  it  is  only  within  recent  years  that  their  true  consti- 
tution has  been  determined.  In  Russian  oils  they  are  the  princi- 
pal constituents,  and  according  to  C.  F.  Mabery  and  E.  J. 
Hudson  they  also  predominate  in  California  petroleum. 


TABLE  IV. — SO-CALLED  "  OLEFINES"  ISOLATED  FROM  PETROLEUM 


Name 

Formula 

Melting 
point 

Boiling 
point 

1.  Gaseous: 
Ethylene  .  . 

C2H4 

—  103 

Propylene  

C3H6.. 

—   18 

Butylene  .... 

C4H8 

—     5 

2.  Liquid: 
Amylene  

C  H 

- 

+  35 

Hexylene  

68 

Heptylene  

98 

Octylefte.  . 

124 

Nonylene  
Decylene  

CgHig..  . 

P    TT 

153 
172 

Undecylene  
Duodecylene  

CnH22.  . 

195 
216 

Tridecylene  . 

r  TT 

232  7 

Cetene  

C16H32 

275 

3.  Solid: 
Cerotene  

65-66 

Melene  

CsoHeo-  . 

62 

Members  of  the  series  from  C7Hi4  to  Ci5H30  were  isolated  from 
the  California  material.  Mabery  and  S.  Takano  also  found  that 
Japanese  petroleum  consisted  largely  of  CnH2n  hydrocarbons. 
Other  similar  occurrences  are  recorded  in  the  treatises  of  Hofer 
and  Redwood. 


22 


PRACTICAL  OIL  GEOLOGY 


The  series  CnH2n-2  is  often  called  the  acetylene  series,  after 
its  first  member,  acetylene,  C2H2.  The  lower  members  of  this 
series  have  not  been  found  in  petroleum,  but  several  of  its  higher 
members  are  characteristic  of  oils  from  Texas,  Louisiana,  and 
Ohio.  In  oil  from  the  Trenton  limestone  of  Ohio,  Mabery  and 
O.  H.  Palm  found  hydrocarbons  having  the  composition  Ci9H36, 
C2iH4o,  C22H42,  and  C24H4e.  With  these  compounds  were  also 
members  of  the  next  series,  CnH2n_4 — namely,  C23H42,  C24H44, 
and  C25H46.  In  petroleum  from  Louisiana,  C.  E.  Coates  and 
A.  Best  found  the  hydrocarbons  Ci2H22  and  Ci4H2e.  These, 
together  with  Ci6H30,  were  also  separated  by  Mabery  from  Texas 
oils.  These  oils  are  furthermore  peculiar  in  containing  free 
sulphur,  which  separates  in  crystalline  form. 

Table  V  shows  the  average  chemical  composition  of  a  number 
of  petroleums. 

Table  VI  gives  the  commercial  values  of  the  same  petroleums. 


ANALYSES 


TABLE  V. — ELEMENTAL  ANALYSES1 


Nos. 

Field 

Specific 
gravity 
at  15°  C. 

Heating 
value  per 
gram 

Hydro- 
gen 

Carbon 

Nitro- 
gen 

Sul- 
phur 

Unde- 
ter- 
mined 

545 

Kern  River 

Calories 

Per 

cent. 

Per 
cent. 

Per 

cent. 

Per 

cent. 

Per 
cent. 

composite2   .  . 

0.9670 

10,312 

11.27 

86.36 

0.74 

0.89 

0.74 

535 

Coalinga  

.9505 

10,400 

11.30 

86.37 

1.14 

.60 

.59 

542 

McKittrick.  .  . 

.9600 

10,186 

11.41 

86.51 

.58 

.74 

.76 

543 

Midway  

.9580 

10,314 

11.61 

86.58 

.74 

.82 

.25 

544 

Sunset  

.9705 

10,233 

11.37 

85.64 

.84 

1.06 

1.09 

1  The  samples  were  dried  by  filtering  twice  through  about  2  cm.  of  anhy- 
drous sodium  sulphate  before  analysis. 

2  See  Table  VI. 


PETROLEUM 


23 


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24 


PRACTICAL  OIL  GEOLOGY 


TABLE  Via. — AVERAGE    ANALYSES    SHOWING    COMMERCIAL    VALUES    OF 

OKLAHOMA  OILS 


Location 

Specific 
gravity, 
(at  15° 
C.) 

Degrees 
Baum6, 

(60°  F.) 

Calories 
per 
gram 

B.t.u. 
per 
pound 

Viscosity 
at  20°  C. 
(Engler 
scale) 

Water 
(per 
cent.) 

Sul-, 
phur 
per 
cent.) 

Avant  . 

0  8617 

32  49 

10  828 

19  490 

2  ^ 

l 

0   17 

Bald  Hill  

.8465 

35  40 

10905 

19  629 

2  3 

0  1 

17 

Bartlesville  
Bigheart  
Checotah  

.8604 
.8547 
.8610 

32.71 

35.58 
32  60 

10,883 
10,904 
10  910 

19,585 
19,589 
19  638 

2.3 
2.3 
3  5 

.0 

1 

.14 
.16 
H 

Cleveland  
Collinsville-  Clare- 
more. 
Gushing 

.8388 
.8585 

8389 

36.94 
33.10 

37  00 

10,921 
10,846 

10911 

19,658 
19,524 

19  639 

1.7 

2.6 

2  0 

.0 
.0 

o 

.21 
.20 

07 

Flat  Rock  
Glenn  Pool  
Gotebo  

.8635 
.8445 
.8595 

32.14 
35.83 
32  89 

10,804 
10,879 
10,925 

19,448 
19,582 
19,665 

3.0 
1.8 
2  9 

.0 

.2 
i 

.26 
.28 
25 

Hamilton  Switch  .  .  . 
Henrietta  
Hominy  Creek  
Madill... 

.8439 
.8720 

.8585 
8504 

35.92 
30.55 
33.09 
34  64 

10,907 
10,761 
10,838 
10893 

19,633 
19,370 
19,508 
19  608 

2.0 
3.3 

2.7 
3  7 

.0 

J 

.1 
o 

.18 
.35 
.20 
16 

Mounds  

.8635 

32.14 

10,826 

19,488 

3  5 

0 

22 

Musk  ogee  
Nelagony  
Nowata  

.8304 
.8615 
.8525 

38.60 
32.51 
34.22 

11,009 
10,827 
10,920 

19,817 
19,489 
19,656 

1.5 

2.4 
1  8 

.0 

i 

1 

.10 
.19 
14 

Okmulgee  
Oresa  
Osage  City  
Pawhuska  
Ponca  City 

.8530 
.8665 
.8472 
.8710 
8144 

34.13 
31.58 
35.30 
30.73 
41  91 

10,850 
10,836 
10,879 
10,807 
10998 

19,531 
19,506 
19,506 
19,453 
19  797 

2.6 
3.0 
2.0 
6.6 
1  2 

.1 

.3 
.0 
.1 

o 

.20 
.18 
.24 
.23 
10 

Red  Fork  
Salt  Creek  
Sapulpa  .  .  . 

.8457 
.8511 
8635 

35.57 
34.52 
32  14 

10,928 
10,881 
10826 

19,670 
19,585 
19  486 

1.9 
2.6 

2  7 

.0 
.0 

o 

.24 
.17 
25 

Schulter  

.8600 

32.84 

10,840 

19,513 

2  8 

.0 

.23 

Turley  

.8772 

29  67 

10,790 

19,422 

7  2 

o 

23 

Wheeler  

9166 

22  76 

10554 

18998 

40  2 

6 

1  20 

Grand  average  

.8544 

33.96 

10,870 

19,567 

3.9 

.0 

.23 

1  Trace. 


PETROLEUM 


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28 


PRACTICAL  OIL  GEOLOGY 


A  complete  analysis  of  several  petroleums  are  presented  below. 

ANALYSIS  OF  SULPHUR  MOUNTAIN   PETROLEUM  COMPANY 
VENTURA  COUNTY,  CALIFORNIA 


Gravity 

Viscosity  at  60°  F. 
Viscosity  at  185°  F. 
Flash  point 
Sulphur 
Thermal  value 


17.6°  Baume 
81.15  Redwood 

2.65  Redwood 
Below  60°  F. 

1.46  per  cent. 
18,551  B.t.u. 


Distillation  Results 

Sample  of  200  c.c.  distilled  in  glass  with  steam. 


Below  212°  F. 

212°  to  302° 

302°  to  392° 

392°  to  482° 

482°  to  572° 

572°  to  grade  D  asphalt 

Asphalt 

Water  and  loss 


3.1  per  cent. 
4.8  per  cent. 
8.1  per  cent. 
7.4  per  cent. 
8.8  per  cent. 
20.0  per  cent. 
25.0  per  cent,  grade  D 
2.8  per  cent. 


Gravity 
58.0°  Baume 
50.7°  Baume* 
41.2°  Baum^ 
32.4°  Baume 
28.8°  Baume 
20.5°  Baume 


100.0  per  cent. 


This  corresponds  in  round  figures  to  the  following  commercial 
analysis. 


Gasoline 
Distillate 
Kerosene 
Stove  oil 
Fuel  oil 

Lubricating  stock 
Asphalt  grade  D 
Losses 


61.0°  Baume 
52.0°  Baume 
42.0°  Baume 
34.0°  Baume 
28.0°  Baume 
20.5°  Baume 


2  per  cent. 
6  per  cent. 
8  per  cent. 
6  per  cent. 

30  per  cent. 
20  per  cent. 
25  per  cent. 

3  per  cent. 
100  per  cent. 


PETROLEUM  29 

ANALYSIS  OF  GUSHING,  OKLAHOMA,  OIL 

The  following  tests  made  on  Gushing  crude  oil  by  the  Cosden 
Refining  Company  at  Tulsa  will  also  give  an  idea  of  the  char- 
acter of  the  Gushing  oil. 

RESULT  OF  A  TEST  RUN  ON  30,000  GALLONS  OF  40.9°  BAUME  GUSHING 
CRUDE,  FROM  BARTLESVILLE,  WHEELER,  AND  LAYTON  SANDS 

Crude  benzine 36 . 0 

80  per  cent,  of  this  if  re-run  would  be  finished 
60  per  cent,  gasoline 

Kerosene 20.0 

Gas  oil 10.0 

Wax  distillate 21 .0 

Residuum 9.0 

Layton  crude  with  43.5°  Baume 

Gasoline  60°  to  61°  gravity 50.0 

Water  white  40°  to  41°  gravity 12\5 

Residuum  or  road  base 33 . 5 

Loss 4.0 

Test  of  580  bbl.  of  Gushing  crude  40°  Baume  from  Bartlesville, 
Wheeler,  and  Layton  sands 

207.54  bbl.  crude  benzine,  or  35.78  per  cent. 
96.66  bbl.  water  white  distillate,  or  16.67  per  cent. 
177.68  bbl.  wax  distillate,  or  30.64  per  cent. 
70.37  bbl.  residuum,  or  13.51  per  cent. 
19.7  bbl.  loss,  or  3.4  per  cent. 
Wheeler  crude  41.2°  Baume 

Gasoline  60°  to  61°  gravity 37 . 5 

Water  white  40°  to  41°  gravity 21.0 

Wax  distillate 26.0 

Tar,  or  heavy  residuum 12.0 

Loss 5.5 

Records  obtained  from  the  Superintendent  of  the  Cosden  Refining  Com- 
pany, Tulsa,  Oklahoma. 


CHAPTER  III 

•.. 

STRATIGRAPHY 

Stratigraphy  is  the  detailed  study  of  the  order  of  deposition, 
and  the  relative  ages  of  the  stratified  rock  that  make  up  the 
earth's  crust. 

CLASSES  OF  ROCKS 

All  rocks  that  make  up  the  earth's  surface  belong  to  three 
great  classes: 

(1)  Igneous  rocks,  or  volcanics.  (2)  Sedimentary  rocks,  and 
(3)  Metamorphosed  igneous  and  sedimentary  rocks. 

Igneous  rocks  are  of  many  different  kinds — granites,  syenites, 
basalts,  etc. 

Sedimentary  rocks,  such  as  clays,  shale  sands,  sandstones, 
limestones  and  other  fragmentals  are  derived  from  material 
broken  from  the  igneous  rocks,  and  from  other  sedimentary  rocks. 

Metamorphics, — gneisses,  schists,  slates,  etc.,  are  due  to 
changes  in  both  the  igneous  and  sedimentary  rocks. 

Any  one  prospecting  for  oil,  is  most  interested  in  sedimentary- 
rocks.  These  rocks  are  formed  by  the  breaking  down  of  igneous 
rocks  due  to  the  action  of  rains,  frosts,  winds,  running  water  and 
other  agencies.  Beds  of  material  are  deposited  along  stream 
courses,  along  the  sea  coasts  and  lake  shores.  The  principal 
types  of  sedimentary  formations  are  classified  as  follows:  Con- 
glomerate; sand;  sandstones;  clay;  shales;  and  limestones. 

Gravel — Conglomerate. — Loose  aggregates  of  rounded  or 
water-worn  pebbles  are  called  gravel.  When  pebbles  become 
cemented  together  into  coherent  rocks,  they  form  conglomer- 
ates. Silica,  calcite,  and  limonite  are  the  principal  cements 
that  bind  the  particles  together. 

30 


STRATIGRAPHY  31 

Gravel  and  conglomerate  of  limited  extent  indicate  the  former 
presence  of  swift  streams;  if  of  wide  area  they  suggest  the  ex- 
istence of  sea  beaches  and  the  advance  of  sea  over  land. 

Sand — Sandstones. — The  sediments  finer  than  pebbles  and 
yet  of  noticeable  size,  such  as  beach-sands  or  river-sands,  are 
classed  as  sands.  They  are  generally  made  up  of  quartz  grains. 
In  river  sands,  the  grains  are  angular — in  beach  sands,  rounded. 
Where  sands  are  cemented  by  calcite  and  limonite,  they  form 
sandstones  and  when  by  silica  they  form  quartzites. 

Clays — Shales. — Clay  is  made  up  of  very  fine-grained  particles 
principally  of  aluminous  materials  containing  considerable 
water.  Clays  are  formed  in  deeper  water  than  sands.  Shales 
are  laminated  rocks  made  up  from  hardened  muds,  silts  or  clays, 
as  the  result  of  pressure. 

Limestones. — Limestones  are  formed  principally  of  calcium 
carbonate  resulting  from  the  deposition  of  calcium  salts  in  lake 
beds,  or  the  accumulation  of  corals  and  the  shells  of  other 
marine  organisms,  in  coral  reefs,  or  on  the  ocean  floor. 

All  of  the  classes  of  rocks  above  named  are  modified  to  form 
sub-classes^nd  types  without  number.  For  all  practical  purposes 
the  above  definitions  are  sufficient. 

Deposition. — It  is  an  ascertained  fact  that  the  sea  coasts  of 
the  world  are  either  rising  or  falling  very  slowly.  This  being 
true,  it  is  assumed  that  the  ancient  sea  coasts  were  also  rising  and 
falling.  Indeed,  such  must  have  been  the  case  when  we  find 
great  beds  of  sea-shells  upon  high  mountain  tops.  Suppose  a 
sea  coast  is  sinking  slowly  and  as  the  sedimentary  material  is 
deposited  it  is  constantly  covered  up  by  the  addition  of  matter 
above  it.  In  time,  layers,  hundreds  of  feet  thick,  are  accumu- 
lated. Sometimes  such  layers  or  beds  contain  much  organic 
material  in  them.  Again,  the  coasts  may  rise  slowly  and  in  time 
be  above  water.  At  the  same  time,  the  forces  that  caused  this 
rising  (due,  perhaps,  to  the  contraction  of  the  earth  which  con- 
stantly tends  to  become  smaller  in  diameter)  caused  a  folding  or 
crumpling  of  the  earth's  crust. 

We  thus  have  reasons  both  for  the  accumulations  of  great  thick- 


32  PRACTICAL  OIL  GEOLOGY 

ness  of  material  and  for  the  occurrence  of  various  formations 
above  sea  level.  The  types  of  structure  caused  by  crumpling 
and  the  attendant  changes  are  classified  in  Chapter  IV. 

Source  of  Supply  of  Material. — In  every  region  there  is  a 
certain  part  from  which  the  materials  that  form  the  sedimentary 
rocks  are  derived.  In  most  regions,  igneous  or  volcanic  rocks 
form  the  backbone  of  the  mountain  ranges,  and  it  is  from  these 
that  the  materials  are  derived. 

The  tendency  of  every  bed  is  to  decrease  in  thickness  away  from 
the  source  of  supply.  When  the  source  of  supply  is  known,  it  is 
not  difficult  to  predict  the  thickening  and  thinning  of  beds  and 
the  occurrence  of  conglomerates,  of  sands,  and  of  shales.  Con- 
glomerates form  near  the  source  of  supply,  sands  farther  away, 
and  shales  at  a  still  greater  distance. 

Variations  in  Beds. — The  thickness  of  beds  in  some  districts 
may  increase  or  decrease  greatly  in  the  distance  of  a  mile  or  more. 


Cliff 


XXX       X    \  

><  X   X    X   x    X    X~X> 

xxx   xxx  xS 
x;  X   X  X  x  x  x    X/ 
*  X    X    X    X    X    "X/7 

xx  x  x  x  x  x^£ 
x;X  x  x  x  x  xy 

x^X    X  x    X  X   X/ 
x  X  X   X  x  X  yl       Boulders 
xx  x  x  x  x  A/             Sa,nd 
vX  x    X  x   Xjoo0°o^Ss_            /             Sea     Level 

Y  *•  \  x  [  igneous  Eock 
|o0o°o000o0|  Gravel 
J  Sand 
IL-T^-TZJ  Clay,  Shale 

xx  x  x  x  x^uS^rfiS^orr-?^  —  ^—  _ 

x  -x  -  ><  n^^s^^^^^^- 

Clay 

KXXXXXXXXXX  x^^^?^^^ 

^. 

xxxxxxxxxx  xx  x  >S^^|So-£?^ 

|^^^^v^_^         Shale 

^XXXXXXXXXXXXXX    Xx~§2^~^ 

yXxXXXXXxXxXXXXXXXXXX 

x    ^~^  V 

<XXXXXXXXXXXXXXXXXXXX 

x  x  x  xx~~^e^^^^v^^ 

xXxxxxxxxxXxxxxxxxxxx 

XXXX     *  X     XX    x'^px 

X      XX      yyXXXXXXXVXXXXXX      X 

xxyyyxvxxx  a  * 

FIG.  2. — Illustrates  the  formation  of  sediments  along  a  sea  coast. 

With  some  formations,  such  as  boulders  and  stream  deposits, 
changes  may  occur  in  much  shorter  distances.  Shale  and  sand 
beds  1500  ft.  thick  have  totally  disappeared  in  a  little  over 


STRATIGRAPHY 


33 


5  miles.  .  A  bed  may  start  as  a  conglomerate,  in  f£  mile  change 
to  a  sand,  and  further  on  to  shale.  (See  Fig.  2.) 

Knowledge  of  such  changes  is  of  great  importance  in  determin- 
ing reservoirs  for  oil,  and  in  locating  well  sites  to  obtain  the  most 
productive  wells,  as  will  be  shown  later. 

Where  underground  waters  percolate  through  the  formations, 
and  carry  cementing  material,  conglomerate,  sandstone,  and 
silicified  shale  result.  Where  cementing  material  is  absent,  loose 
gravel  and  soft  shales  occur.  It  is  in  these  gravel  beds,  sands, 
and  shales  that  oil  finds  its  best  resting  place. 

Conformity — Unconformities. — Where  beds  are  deposited  in 
order  without  breaks,  as  in  Fig.  2,  they  are  conformable.  Beds 
B,  Cj  and  Z>,  in  Fig.  3,  are  also  conformable. 


TTrn-i-rrr-: -"^—r-—^^ 


FIG.  3. — Illustrates  conformity,  erosional  and  angular  unconformity 

Where  the  beds  have  been  pushed  above  water  and  exposed  to 
erosion,  then  have  sunk  again  and  other  beds  formed  upon  them, 
one  finds  an  erosional  unconformity,  as  shown  at  A7  in  Fig.  3. 

Where  beds  have  been  deposited,  then  upheaved,  and  folded, 
later  eroded  and  have  then  sunk  again,  and  still  later  beds  depos- 
ited on  them,  a  condition  similar  to  that  at  E  in  Fig.  3  results. 
This  condition  is  called  an  angular  unconformity. 

3 


34 


PRACTICAL  OIL  GEOLOGY 


Fig.  4  illustrates  a  condition  by  no  means  uncommon.  The 
lowest  formation  is  older  than  the  upper  and  was  tilted  before 
the  upper  bed  was  deposited  on  top.  Wells  at  1,  strike  sand  A ; 
at  2,  A  and  B;  at  3  A ;  at  4,  A  and  C. 


FIG.  4. — Illustrates    angular   unconformity,    its    relation    to  migration    of 
petroleum,  also  relation  of  wells  on  such  a  structure. 


FIG.  5.- — Unconformity  between  tilted  Monterey  shale  and  horizontal 
Pleistocene  sand  and  gravel  in  Californiac     (Bull.  322,  tL  S.  G.  S.) 


STRATIGRAPHY 


35 


OVERLAP. — If  beds  are  deposited  along  shore  lines  while  the 
Coast  is  slowly  sinking,  the  first  beds,  A,  laid  down  (see  Fig.  6) 
are  covered  by  material  that  is  in  turn  covered  and  hidden  by 
later  beds,  B,  C  and  D.  The  higher  beds  progressively  lap  over 
the  lower  beds.  Fig.  6  shows  an  overlap  and  also  an  unconform- 
ity between  the  crystalline  rocks  and  the  sedimentaries. 


ii^^-x  x 

*    X    X    X 


X    X 


X    X 


:  x   x 

:-==Z==-»«~X   XXXXXXXXXXXXX 
XXXXXXXXXXXXXXXX 

,.;;^dlliiH«i^iliiii 


FIG.  6. 


RELATIVE  AGES  OF  FORMATION 

Order  of  Superposition. — The  relative  ages  of  beds  depend 
primarily  upon  the  order  of  their  deposition.  Normally  the 
older  beds  -are  beneath  the  younger.  This  simple  law  is  the  basis 
for  determining  the  ages  of  beds.  However,  folding  and  faulting 
may  change  the  position  of  certain  beds  but  not  their  age. 

Hardness  of  Formations. — The  older  the  formations,  the  harder 
they  are,  generally.     Sand  grains  when  cemented  become  sand- 
stones, shales  become  compressed  and  cemented  to  form  slates. 
Jointing  also  becomes  more  pronounced  in  the  older  rocks. 

Lithologic  Similarity. — Many  times  attempts  have  been  made 
to  correlate  or  classify  rocks  in  a  region  according  to  their  color, 
texture,  or  mineral  composition.  Such  means  of  correlating 
do  not  hold  good  under  all  conditions,  and  serious  mistakes 
are  sometimes  made  by  correlating  certain  beds  which  in  reality 
may  be  some  distance  apart.  The  surest  test  of  working  out 
any  structure  is  to  "walkout"  the  beds,  that  is,  follow  one 
known  bed  in  all  its  rising  and  falling.  In  some  places  such 


36  PRACTICAL  OIL  GEOLOGY 

procedure  cannot  be  accomplished,  in  others  it  is  not  necessary 
as  the  structure  can  be  clearly  seen. 

A  sandstone  of  Cretaceous  age  may  look  exactly  like  one  of 
Miocene  age  in  color  and  in  texture.  Sands  of  both  ages  often 
appear  identical.  However,  in  some  cases,  where  marked  dis- 
similarity appears  over  a  district,  certain  formations  may  be 
readily  correlated. 

If  a  thick  red  shale  occurs  in  a  region  it  may  extend  over 
several  hundred  square  miles  and  act  as  a  horizon  marker. 

Conglomerates,  due  to  terrestrial  or  near-shore  conditions, 
are  not  reliable  markers  as  they  may  die  out  within  1  or  2  miles. 

Some  sandstones  are  very  persistent.  They  mark  old  beach 
conditions;  and  often  cover  entire  counties.  Limestones  are  also 
very  persistent  horizon  markers. 

It  is  essential  to  discover  the  peculiarities  of  any  forma- 
tion, for  where  fossil  evidence  is  lacking  such  peculiarities  are 
often  of  value  in  checking  up  formations  within  the  limited  ra- 
dius of  2  to  3  miles. 

Fossils. — The  proper  correlation  of  beds  is  best  accomplished 
by  the  use  of  fossils.  It  has  been  found  that  when  certain 
classes  of  fossils  occur  in  a  stratum,  the  age  of  the  stratum  is  fixed 
definitely  and  its  position  above  or  below  others  is  determined. 
However,  fossil  evidence  must  be  used  very  carefully  and 
only  by  experienced  workers. 

Fossils,  such  as  oyster  shells,  scallops,  gasteropods  and  micro- 
scopic organisms  (foraminifera  and  diatoms),  form  the  chief  basis 
for  stratigraphic  classification. 

Fossils  may  be  of  great  value  to  the  geologist  and  practical 
man,  but  it  is  important,  however,  to  know  the  fossils  that 
are  characteristic  of  certain  horizons. 

In  some  regions  beds  of  known  ages  carry  oil  and  all  others 
may  be  ruled  out.  The  ages  are  determined  by  the  order  of 
superpositions  as  explained  above,  and  by  fossil  evidence. 
Where  beds  are  overturned  as  in  Fig.  7,  fossil  evidence  gives  a 
clue  to  the  true  condition.  The  fossils  in  A  are  younger  than 
those  in  B.  Normally  A  would  overlie  B  as  at  /.  At  //  the 


STRATIGRAPHY 


37 


order  is  reversed  while  at  ///  the  order  is  normal  again.  The 
true  order  of  the  beds  is  then  as  shown  at  /  and  ///.  Other 
evidence,  as  dips  and  curvatures,  generally  bear  out  such  condi- 
tions, and  in  some  cases  actual  folds  in  miniature  show  the 
true  condition. 


in 


FIG.  7. — Illustrates  the  use  of  fossils  in  correlating  formations. 

By  means  of  fossils  one  may  often  correlate  oil-bearing  forma- 
tions many  miles  apart.  Fig.  8  shows  the  application  of  fossil 
evidence--  At  /  oil  occurs  under  the  beds  carrying  certain 
fossils,  as  oysters  and  scallops.  At  II  the  same  fossils  occur 


FIG.  8. — Illustrates  the  use  of  fossils  in  correlating  formations. 

which  would  lead  one  to  believe  that  the  same  oil  strata  under- 
lie the  fossil  beds  occurring  at  this  point. 

Oil  operators  from  Pennsylvania  will  see  certain  formations 
in  California,  say  a  sandstone,  that  looks  very  much  like  a  bed 
in  Pennsylvania.  Immediately  the  operators  will  call  the  bed 


38 


PRACTICAL  OIL  GEOLOGY 


the  same  age,  when  in  reality  there  is  absolutely  no  connection 
between  the  two.  The  California  oil  formations  are  millions 
of  years  later  than  the  Penn- 
sylvania oil  formations.  The 
lowest  California  oil  is  in  the 
topmost  Cretaceous,  and  the 
highest  Pennsylvania  forma- 
tions are  in  the  Carboniferous 
system.  Table  VIII  shows  the 
difference  between  the  East- 
ern and  Western  oil  horizons. 
These  differences  are  worked 
out  by  correlating  the  fossil  evi- 
dence throughout  the  United 
States. 

Geologic  Column. — A  geo- 
logic column  (see  Fig.  9)  is 
often  of  great  assistance.  By 
means  of  such  a  column  one 
knows  the  formations  that 
must  be  penetrated  by  the 
drill,  the  thickness  of  each  for- 
mation, the  depth  to  oil,  etc. 

These  vertical  sections  en- 
able the  contractors  and  drill- 
ers to  choose  drilling  rigs,  and 
to  determine  the  depths,  hard- 
ness of  formations,  etc.  Such 
a  column  is  merely  a  graphic 
representation  of  the  stratig- 
raphy of  a  region.  One  col- 
umn does  not  hold  good  over 
a  very  large  area,  so  new  sec- 
tions  must  be  made  for  dif- 
ferent  parts  of  a  region  to 
take  into  account  local  changes  in  thickness,  character,  etc. 


G=>    <=,         <__.,     <=. 
*-^~*      CO    O     o  a 

^200 

Gravel 

~      ~  _-_-_-. 

•  175 

Gray 

975  'J 
Fernando 

—  -  —  rrzL. 



lied 

rr^jrrzL^l 

Shale 
Clay 

\   Lnc 
I  500 

onforinity 

Conglomerate 
Granite 
Pebbles 

~  ~^  ^^  - 

U' 

Water 
Sand 

I860' 

^—rE^-^rL 

loo 

Petroliferous 



UOO 

Shale 

.  . 

(Brown) 

:v:V".::-:^":V:.-;-'-':V. 

1.200 

Oil  -Sandstone 

/ 

^B 

I  250 

{  Unc 
'175' 

Oil  -Sand 
(Coarse) 
onformity 
Blue 
Sand  Stone 

.  —  .  —  . 

• 

300 

Black  Shale 

725' 
Vaqueros 

150' 

Oil  Stained 
Oil  Sand 

(.100 
\  Un 
7150' 

Blue  S.S. 
onformity 
Blue  Shale 

450' 

-:;'v'::^v^0iv:!:::- 

}  100 

Blue  S.S. 

Sespe 

1 
^200 

<Un 

Oil  Shale 
onformity 

-Tn^-T^irm^r 

"9^'-£/Jta& 

Oil  Shales 

800' 
Tejon 

^B 

300 

Sands 

Sand 
Stones 

2000' 

^Z^rr"-    __^_ 

Gr 

en  Shales 

Martinez 

STRATIGRAPHY  39 

Geological  Names  (After  J.  F.  Kemp). — "In  the  advance  of 
geological  science  the  standpoints  from  which  the  strata  forming  the 
earth's  crust  are  regarded  necessarily  change,  and  new  points  of  view  are 
established.  In  the  last  few  years  two  have  become  especially  promi- 
nent, and  there  are  now  two  sharply  contrasted  positions  from  which  to 
obtain  a  conception  of  the  structure  and  development  of  the  globe. 
The  first  is  the  physical,  the  second  is  the  biological. 

"  For  example,  we  consider  the  surface  of  the  earth  as  formed  by  rocks, 
differing  in  one  part  and  another,  and  these  different  rocks  or  groups  of 
rocks  are  known  by  different  names.  The  names  have  no  special 
reference  to  the  animals  found  in  them,  but  merely  indicate  that  series 
of  related  strata  form  the  surface  in  particular  regions. 

"  On  the  other  hand,  the  rocks  are  also  regarded  as  having  been  formed 
in  historical  sequence,  and  as  containing  the  remains  of  organisms 
characteristic  of  the  period  of  their  formation.  They  illustrate  the 
development  of  animal  and  vegetable  life,  and  in  this  way  afford 
materials  for  historical-biological  study.  In  the  original  classification, 
the  biological  and  historical  considerations  are  all-important. 

"But  when  once  the  rocks  are  placed  in  their  true  positions  in  the 
scale,  and  are  named,  these  considerations  for  many  purposes  no  longer 
concern  us.  The  formations  are  regarded  simply  as  members  in  the 
physical  constitution  of  the  outer  crust. 

"The  International  Geological  Congress  held  in  Berlin  in  1885 
expressed  these  different  points  of  view  in  two  parallel  and  equivalent 
series  of  geological  terms,  which  are  tabulated  below.  They  are  now 
very  generally  adopted.  For  clearness  in  illustration  the  equivalent 
terms  employed  by  Dana  are  appended. 

Biological  Terms  Physical  Terms  Dana's  Terms  Illustrations 

Era                           Group                 Time  Paleozoic 

Period                      System                Age*  Devonian^ 

Epoch                      Series                   Period  Hamilton 

Age*                         Stage                   Epoch  Marcellus 

"The  United  States  Geological  Survey  divides  as  follows:  Era  and 
System,  Period  and  Group,  Epoch  and  Formation.  In  considering  the  ore 
deposits  of  the  country,  we  employ  only  the  physical  terms.  We 
understand,  of  course,  the  chronological  positions  of  the  systems  in 

*In  making  reports  to  the  layman  the  term  "Age"  is  probably  the 
easiest  understood. 


40  PRACTICAL  OIL  GEOLOGY 

historical  sequence,  but  it  is  of  small  moment  in  this  connection  what 
may  be  the  form  of  life  enclosed  in  them.  The  purely  physical  character 
of  the  rocks — whether  crystalline  or  fragmental;  whether  limestone, 
sandstone,  granite  or  schists;  whether  folded,  faulted  or  undisturbed — 
are  the  features  on  which  we  lay  especial  stress.  In  all  the  periods  the 
same  sedimentary  rocks  are  repeated,  and  in  the  hand  specimens  it  is 
almost  always  impossible  to  distinguish  those  of  different  ages  from  one 
another." 

DEFINITIONS  OF  FORMATION  AND  MEMBER 

(After  Snyder) 

"The  systems  of  rocks  are  divided  into  smaller  groups  known  as 
formations.  A  formation  is  defined  as  a  mappable  unit,  that  is  a  layer 
of  rock  or  group  of  layers  which  extends  entirely  across  the  area  under 
consideration  and  has  sufficient  width  of  outcrop  to  be  mapped.  For- 
mations may  consist  of  single  ledges  or  beds  of  rock,  but  commonly 
are  made  up  of  two  or  more  closely  related  beds. 

The  separate  beds  are  sometimes  called  members.  Thus,  the  prin- 
cipal oil  sands  of  the  main  (Oklahoma)  oil  and  gas  field  may  be  con- 
sidered as  members  of  the  Cherokee  formation.  The  formations  and 
members  are  usually  named  from  the  place  where  they  are  best  developed 
or  where  they  were  first  studied.  The  Pitkin  limestone  which  out- 
crops east  of  Muskogee  and  Ft.  Gibson  is  an  example  of  a  formation 
consisting  of  only  one  kind  of  rock.  The  Ft.  Scott  formation,  known 
to  the  drillers  as  the  Oswego,  is  usually  called  the  Ft.  Scott  limestone, 
but  really  consists  of  two  limestones  separated  by  a  shale.  Formations 
may  vary  in  thickness  from  a  few  feet  to  thousands  of  feet.  Thus  the 
Chattanooga  shale  in  northeastern  Oklahoma  is  not  over  50  feet 
thick  while  the  Arbuckle  limestone  in  the  Arbuckle  Mountains  is 
over  5,000  feet  thick.  Formation  names  are  a  great  convenience 
since  they  provide  a  means  of  designating  certain  beds  of  rock  with- 
out repeating  extended  descriptions.  They  are  necessarily  used  ex- 
tensively in  the  description  of  the  geology  of  any  region." 

The  accompanying  geological  charts,  and  tables  show  the  pro- 
ductive oil  horizons  of  the  western  United  States. 


Cenozoic .  . 


Mesozoic 


STRATIGRAPHY 
TABLE  VII. — GEOLOGICAL  ERAS  AND  THEIR  SUBDIVISIONS 

f  Present 
Pleistocene 
Pliocene 
Miocene 
Oligocene 
Eocene 

Transition — Aprapahoe  and  Denver 
Upper  Cretaceous 

Lower  Cretaceous        Comanche  or  Shastan 
Jurassic 


Paleozoic . 


Triassic 

Carboniferous 

Devonian 
|  Silurian 
I  Ordovician 
[  Cambrian 


Permian 

Pennsylvanian 

Mississippian 


Great  unconformity 

[  Keweenawan 
|  Unconformity 

Proterozoic  .-T j  Animikean 

Unconformity 


Huronian 
Great  unconformity 


Archeozoic..  . 


Archaean 


Complex 


Great  granitoid  series  intrusive  in  the  main 

Lauren  tian 
Great  schist  series  Mona,  Kibchi,  Keewa- 

tin,  Quinneseo. 


42 


PRACTICAL  OIL  GEOLOGY 


TABLE  VIII. — GEOLOGICAL  FORMATIONS  OR  "SANDS"  IN  WHICH  OIL  AND 
GAS  ARE  FOUND  IN  THE  UNITED  STATES  AND  CANADA 

This  chart  was  prepared  with  a  view  of  showing  the  various  oil  and  gas  sands  with  reference 
to  their  age  and  position  in  the  stratified  rocks  forming  the  earth's  crust.  Owing  to  the 
fact  that  some  of  the  oil  fields  have  not  been  given  thorough  geological  study  and  also  that 
geologists  are  not  yet  certain  regarding  the  age  of  several  of  the  formations,  this  chart  is 
of  course  approximated.  Dotted  lines  indicate  points  at  which  uncertainty  exists. 


Era 

Geological 
system 

Geological 
series  or 
group 

Producing   forma- 
tion or  sand 

Correlation 

CENOZOIC 

Quaternary 

Recent  series 

Alluvial  deposits 

Beaumont,    Texas,    and 
Jennings,  Louisiana. 

Tertiary 

Pliocene 

Tulare  formation 

Lost  Hills,  California. 

<n 

•c 

\ 

o 

1 
3 

Upper 
Miocene 

Jacalitos  formation 

Coalinga,  California. 

McKittrick    forma- 
tion 

McKittrick-Sunset,  Cali- 
fornia. 

Fernando  formation 

Santa  Clara  River  &  Los 
Angeles,  California. 

Middle 
Miocene 

Monterey  shale 

Santa  Maria,  California. 
Summerland,  California. 

Puente  formation 

Los  Angeles,  California. 
Salt  Lake  District;  Cali- 
fornia. 

Lower 
Miocene 

Vaqueros  sandstone 

Coalinga,  California. 
McKittrick-Sunset,  Cali- 
fornia. 
Santa  Clara  River,  Cali- 
fornia. 
Parkfield,  California. 

I  MESOZOIC 

Eocene  series 

Sespe  formation 

Sespe  Fields,  California. 
Santa  Clara  River,  Cali- 
fornia. 

Tejon 

Coalinga,  California. 

Topa  Topa 

Santa  Susana,  California 
Sespe  Fields,  California. 

Cretaceous 

Upper 
Cretaceous 

Chico  formation 

Coalinga,  California. 

Mancos  shale 

Colorado. 
Lander   &   Wind    River, 
Wyoming. 

Dakota  sandstone 

North  Dakota. 
Alberta,  Canada  (Gas). 

Webberville   forma- 
tion 

Corsicana,  Texas. 

Aspen  formation 

Spring  Valley,  Wyoming. 

Colorado  formation 

Big  Horn  Basin,  Wyom- 
ing. 

Wall     creek     sand- 
stone     (Lentil     of 
Benton  shale) 

Salt  Creek,  Wyoming. 

Nacatoch  sand 

Caddo,  Louisiana  (Gas). 

Woodbine  sand 

Caddo,  Louisiana  (Oil). 

Lower 
Cretaceous 

Trinity  sand 

Medill,  Oklahoma. 

Jurassic 

Sundance  formation 

N.  E.  Wyoming. 

Triassic 



Chugwater     forma- 
tion 

Wyoming. 

STRATIGRAPHY 


43 


TABLE  VIII. — (Continued)      (OKLAHOMA) 


Era 

Geological 
system 

Geological 
series  or 
group 

Producing   forma- 
tion or  sand 

Correlation 

Blackwell 

Sand 

Sand 

Newkirk 

Elgin  ss.  at  Cleveland 

Sand 

Sand 

Ponca 

Musselman 

Sand 

Sand 

Layton  of  Gushing 

Peoples  of  Cleveland 

Wayside 

Layton  of  Cleveland 
Jones-McEwan 

o 

§ 

| 

Big  lime 

g 

I 

d 

Cleveland 

8 

§ 

| 

Peru 

53 

04 

53 
o 

1 

Oswego  lime 

Wheeler 

Lower  Wheeler 

Peters 

Squirrel 

Bixler-?  Peru 

Skinner 

Pink  lime 

Red  Fork 

Namira 

Sand 

Bartlesville 

Glenn 

Sand 

Tucker 

Burgess-Squaw 
Taneha  Meadows 

Sand 

Butcher 

Lost  city,  96th  meridian 
Rhodes,  Colbert 

44 


PRACTICAL  OIL  GEOLOGY 


TABLE  VIII. — (Continued)     (OKLAHOMA) 


Era 

Geological 
system 

Geological 
series  or 
group 

Producing  forma- 
tion or  sand 

Correlation 

I 

a 
1 
1 

i 

Mounds 

Sapulpa 

Boone  lime 

Main  Mississippi  lime 

Sand 

Pitkin  lime 

TABLE  VIII. — (Continued)     (S.  E.  OKLAHOMA) 


PALEOZOIC 

§ 
a 

O 

Pennsylvanian 

Sand 

Sand 

Oswego  lime 

Calvin  sandstone 

Sond 

Gas  of  Morris 

Glen-Bartlesville 

Sand 

Sand 

Booch 

Second  Booch 

Sand 

Sand 

Mounds 

Morris 

Sapulpa 

Glen  of  Morris 

Sand 

Sand 

is 

11 

S-1 

Fields 

Muskogee-Boynton 
Black-deep  sand 

Sand 

Sand 

STRATIGRAPHY 


45 


TABLE  VIII. — GEOLOGICAL  FORMATIONS  OB  "SANDS"  IN  WHICH  OIL  AND  GAS 

ARE  FOUND  IN  THE  UNITED  STATES  AND  CANADA  (Continued) 


Era 

Geologica 
system 

1 

Geological 
series  or 
group 

Producing  forma- 
tion or  sand 

Locality  where 
productive 

Approxi- 
mate depth 
below 
Pittsburg 
coal.     Feet 

Permian  se- 
ries. 

Upper      coal 

Embar  formation 

Lander,  Wyoming. 



Goodridge  sand 

Bluff,  Utah. 

Connellsville  sand 

West  Virginia. 

40 

Middle     coal 

Morgantown  sand 

West  Virginia. 

80 

Macksburg     sand- 
stone 

S.  E.  Ohio. 

200 

H 

First  Cow  Run  sand 

S.  W.  Penna.  &  W.  Va. 

320 

K 

a 
s 

Middle     Cow     Run 
sand. 

S.  W.  Penna.,  W.  Va.  & 
S.  E.  Ohio. 

450 

> 
">, 
2 

Lower      coal 
measures. 

Lower      Cow     Run 
sand 

S.  W.  Penna.,  W.  Va.  & 
S.  E.  Ohio. 

600 

c 

Bridgeport  sand 

Bridgeport,  111. 

U 

§ 

Carbonif- 
erous 

700     and     800     ft. 
Macksburg  sands 

S.  W.   Penna.,   W.   Va., 
S.  E.  Ohio  &  Ky. 

850  &  925 

H 
$ 

Salt  sands  1 
Gas,  sand  / 

S.   W.   Penna.,   W.   Va., 
S.  E.  Ohio  &  Ky. 

950  to  1080 

Pottsville 

Cherokee       sand- 

Kansas &  Oklahoma. 

group. 

stones 

Buchanan      sand- 
stone 

Casey  &  Robinson  (400 
ft.)  111.,  and  Princeton, 
Ind. 

Benoist  sand 

Sandoval,  111. 

I11.&  Oakland  City,  Ind. 

1 

Keener  sandstone 

S.  E.  Ohio  &  W.  Va. 

1275 

s 

c 

99 

Big  Injun  sand  \ 
Squaw  sand         J 

S.    W.    Penna.,  W.   Va., 
S.  E.  Ohio  &  Ky. 

1340 
1425 

C. 

1 

Pocono 

Berea  grit 

S.  W.  Penna.,  W.  Va., 
S.  E.  Ohio  &  Ky. 

1700 

J 

r? 

First,     100     ft.     or 
Gantz  sand 

W.  Penna.,  W.  Va.  &  S. 
E.  Ohio. 

1850 

50  ft.  sand 

W.  Penna.  &  W.  Va. 

1885 

Second     or     30     ft. 
sand 

W.  Penna.  &  W.  Va. 

2000 

46 


PRACTICAL  OIL  GEOLOGY 


TABLE  VIII. — GEOLOGICAL  FORMATIONS  OR  "SANDS"  IN  WHICH  OIL  AND 
GAS  ARE  FOUND  IN  THE  UNITED  STATES  AND  CANADA  (Continued) 


Era 

Geological 
system 

Geological 
series  or 
group 

Producing  forma- 
tion or  sand 

Locality  where 
productive 

Approxi- 
mate depth 
below 
Pittsburg 
coal.    Feet 

Stray     or     Bowlder 
sands. 

W.  Penna.  &  W.  Va. 

2050 

Third     or     Gordon 

W   Pa     W    Va   &  Ohio 

2130 

sands. 

Fourth,     fifth     and 
sixth  sands 

S.  W.  Penna.  &  W.  Va. 

2200,    2260 
&  2590 

First,     second     and 
third  Warren  sands 

N.  W.  Penna. 

2700,    2815 
&2900 

Tiona  sand 

N.  W.  Penna. 

2950 

Speechley  sand 

N.  W.  Penna. 

3020 

Cherry  Grove  sand 

N.  W.  Penna.  &  W.N.Y. 

3150 

Bradford  sand 

N.  W.  Penna.  &  W.N.Y. 

3460 

Elk  County  sands 

N.  W.  Penna.  &  W.N.Y. 

3650 

Hamilton  formation 

Petrolia    &    Oil   Springs, 
Ontario. 

5330 

U 

§ 

Corniferous       lime- 
stone 

N.  E.  &  Central  Ohio.W 
New  York  &  Ontario. 

5625 

w 
)J 

•< 

PH 

Niagara 
group. 

Oriskany  sandstone 

New   York,    So.    Ind.    & 
Ont. 

5660 

Guelph  limestone 

Ontario  &  W.  New  York. 

5700 

Niagara  limestone 

W.   New   York,   Ontario 
&  Indiana. 

5820 

Clinton  limestone  ) 
Clinton  sandstone  } 

Cen.    Ohio    &    Welland 
Co.,  Ontario. 

5985 
6025 

Medina  red  sand-  \ 
stone 
Medina    white 
sands                       ) 

W.  New  York  &  Wel- 
land Co.,  Ontario. 

6085 
6200 

Trenton    limestone, 
upper. 

N.  W.  Ohio,  Ind.  &  Ky. 

8700 

Trenton    limestone, 
lower 

N.    W.    Ohio,    W.    New 
York  &  Ontario. 

9200 

N    Y  ,    Ga.,   Ala  &   On- 

Potsdam sandstone 

tario. 

Quebec  group 

Newfoundland. 
New  Brunswick. 

9230 

STRATIGRAPHY 


47 


STRATIGRAPHICAL    DISTRIBUTION    OF   PETROLEUM   PRODUC- 
TION TO  1913 

It  is  interesting  to  note  the  ages  that  produce  the  oil  of  America. 
The  table  presented  below,  after  Clapp  (Petroleum  and  Natural 
Gas  Resources  of  Canada,  Vol.  I)  shows  which  ages  have  been 
productive. 


Tertiary 

Upper  Cretaceous. . 

Pennsylvania!! 

Mississippian 

Upper  'Devonian. .  . 

Devonian 

Ordovician . . 


1,935,763,780  bbls. 

42,548,025  bbls. 

343,843,256  bbls. 

726,815,070  bbls. 

540,304,235  bbls. 

14,099,053  bbls. 

318,095,570  bbls. 


California,  Gulf  Coast;  Foreign 
except  Canada. 

Marion  Co.,  Corsicana  to  Powell, 
Texas;  Wyoming;  Colorado.  » 

Electra  and  Henrietta,  Texas; 
Oklahoma;  Kansas. 

Illinois;  one-half  of  the  Appala- 
chian field. 

One-half  of  the  Appalachian  field. 

Canada. 

Lima-Indiana. 


NOTE. — In  Oklahoma  there  is  some  question  as  to  whether  or  not  the 
Permian  is  an  oil-producing  horizon  of  importance.  It  is  certainly  an  impor- 
tant gas  producing  horizon  but  the  petroleum  side  has  not  been  established 
definitely.  ^The  recent  find  at  Garber,  Oklahoma,  is  apparently  Permian, 
close  to  the  Pennsylvanian  contact. 


CHAPTER  IV 

STRUCTURAL  GEOLOGY 
OIL  FIELDS  ON  FLANKS  OF  GREAT  MOUNTAIN  UPLIFTS 

One  great  truth  that  must  be  emphasized  is  the  occurrence  of 
oil-fields  on  the  flanks  of  the  great  centers  or  lines  of  uplifts. 
Thus  all  the  American  oil-fields  at  least,  and  most  European 
fields,  occur  in  the  folded  regions  contiguous  to  centers  or  lines  of 
disturbance.  The  centers  of  these  uplifts  show  the  older  granitic 
rocks,  and  necessarily  do  not  carry  oil,  but  minor  folds  affecting 
the  sedimentary  rocks  form  favorable  conditions  for  accumula- 
tion. The  California  fields  occur  both  on  the  east  side  of  the 
Coast  range,  and  the  west  side  of  the  Sierras  Nevada.  Again, 
the  Utah  fields,  Wyoming  fields,  and  Colorado  fields  occur  on  the 
flanks  of  the  Rocky  Mountains.  The  north  Oklahoma  and  the 
Kansas  fields  occur  on  the  west  flank  of  the  Ozark  uplift,  the 
western  Illinois  fields  be  on  the  east  flank  of  the  Ozark  uplift. 

The  LaSalle  uplift  affects  the  central  Illinois  and  Indiana  oil- 
fields. The  Cincinnati  uplift  controls  part  of  Indiana,  Ohio  and 
Kentucky.  The  West  Virginia  and  east  Pennsylvania  fields 
occur  on  the  West  side  of  the  Alleghenies.  The  southern  Okla- 
homa oil-fields  center  around  the  Arbuckle  uplift.  The  Sabine 
uplift  controls  the  northwestern  Louisiana  fields,  and  the  Burk- 
burnett  uplift  in  Texas  controls  the  north  Texas  fields.  In 
Mexico  the  oil  fields  occur  on  the  eastern  flank  of  the  Mountains. 

Nearly  every  oil  field  in  the  world  occurs  in  close  relationship 
to  some  earth  curve  or  fold.  Underground  structure  is  one  of 
the  most  important  features  of  oil-field  geology.  So  much  de- 
pends on  favorable  structure  that  a  careful  study  of  the  various 
types  of  oil-field  structure  is  necessary.  Below  is  a  classification 
that  is  sufficient  for  all  practical  purposes: 

48 


STRUCTURAL  GEOLOGY  49 


Anticlines . . 


Syiiclines. . . 


f  Single 

[  Compound 

f  Single 

\  Compound 


Symmetrical 

Asymmetrical 

Overturned 

Symmetrical 

Asymmetrical 

Overturned 


3.  Monoclines ...  Terraces 

4.  Combinations  of  1,  2  and  3 

5.  Domes. 

(a)  Anticlinal 
(6)  Saline 
(c)   Volcanic 

6.  Faulted  forms  of  any  of  the  above. 

7.  Stratigraphic  forms — lenses,  fracture  planes,  etc. 

Every  fold  is  part  of  an  earth  curve  and  must  be  considered  as 
continually  changing  in  dip  or  slope.  The  cause  of  folding  is 
problematic.  It  is  thought  to  be  the  result  of  the  contraction  of 
the  earth's  surface  due  to  internal  cooling.  In  places  the  crust 
of  the  earth  is  forced  by  folding  into  arches,  and  sags  or  basins. 
The  results  of  such  folding  are  structures  called  anticlines,  syn- 
clines,  domes  and  monoclines.  Breaks  or  faults  may  affect  all  the 
above  forms  and  make  still  more  complicated  structures. 

Where  masses  of  igneous  rocks  force  the  strata  upward,  folds 
very  similar  to  domes  are  formed.  Volcanic  necks  or  plugs  may 
thus  lift  the  formations  around  them,  forming  arched  structures 
that  are  important  factors  in  the  accumulation  of  petroleum. 
Another  form  of  arching  such  as  the  Saline  domes  of  Texas  and 
Louisiana  is  thought  to  be  due  to  recrystallization  of  salt  masses. 
The  folds  generally  decrease  with  depth.  Folding  is  near-sur- 
face phenomenon,  as  is  often  noticed  the  folds  become  more  con- 
tracted toward  the  center  and  flatten  with  depth. 

1.  Anticlines. — The  distinction  between  anticlines  and  domes 
is  at  present  loosely  drawn.  In  this  book  anticlines  will  be 
differentiated  from  domes  as  follows: 

An  anticline  is  a  long,  relatively  narrow  fold  with  the  dips  or 
slopes  of  its  sides  inclining  away  from  a  line  of  folding  called  an 
axis.  Such  a  fold  will  eventually  disappear  due  to  gradual 


50 


PRACTICAL  OIL  GEOLOGY 


flattening  or  to  faulting,  merging  with  other  folds,  etc.  When  the 
fold  flattens  out,  the  ends  of  the  fold  plunge  or  dip  along  the  line 
of  the  axis  resulting  in  what  is  designated  a  plunging  anticline. 
An  anticline  takes  its  type  name  from  its  cross-section.  Fold- 
ing is  not  only  along  a  plane  vertical  or  inclined  to  the  horizon, 

but  is  also  sinuous  on  the 
surface.  Where  folds  curve 
sharply  the  beds  on  the  inside 
of  the  curve  are  compressed; 
those  on  the  outside  are  under 
tension.  This  results  in  local- 
izing the  oil  at  those  portions 
of  the  fold  which  are  opposite 
the  point  of  greatest  compres- 
sion. 

The  simple  anticline  has 
but  one  high  place  or  apex. 
If  two  or  more  high  places 
form  on  the  long  fold  such 
high  places  are  designated 
anticlinal  domes.  The  low 
places  on  the  anticlines  be- 
tween such  domes  are  called 
" saddles."  Other  names  for 
anticlinal  domes  and  saddles 
are  " structural  highs"  and 
"  structural  lows"  respec- 
tively. A  fuller  discussion  of 
anticlinal  domes  is  given  later 
in  this  work.  Two  other 
types  of  domes  are  also  found  which  will  be  discussed  later. 

Anticlines  are  of  many  forms  of  types:  Symmetrical  anti- 
clines are  those  anticlines  in  which  the  inclinations  or  dips  on 
both  sides  of  the  axis  are  equal.  (See  Fig.  10  a,  b,  and  c.) 

Asymmetrical  or  inclined  anticlines  occur  when  one  of  the 
limbs  or  flanks  has  a  greater  dip  than  the  other.  (See  Fig.  lla, 


FIG.  10. — Forms  of  symmetrical  anti- 
clines. 


STRUCTURAL  GEOLOGY 


51 


116  and  12.)  Asymmetrical  anticlines  are  the  most  common  type 
of  fold.  Folds  are  overturned  when  the  axes  of  the  folds  fall 
over,  as  in  Fig.  14  also  as  in  Fig.  7  in  Chapter  III. 

Isoclines  belong  to  a  peculiar  type  of  symmetrical  anticlines. 
Such  folds  are  not  very  common  but  occasionally  occur. 

Compound  anticlines  consist  of  a  system  of  parallel  anti- 
clines which  often  cover  a  large  area.  The  California  and  Penn- 
sylvania oil  fields  clearly  illustrate  this  condition. 


Dry  Hole 


FIGS,  lla  and  116. — Types  of  asymmetrical  anticlines. 

2.  Synclines. — A  syncline  is  a  structure  the  reverse  of  an 
anticline,  and  receives  its  name  because  its  beds  incline  toward  a 
common  central  line. 

Synclines  are  as  varied  as  anticlines,  and  for  every  anticline 
one  will  nearly  always  find  a  similar  syncline.  Three  examples 
of  synclines  are  shown  in  Figs.  15,  16  and  17. 


\ 

52 


PRACTICAL  OIL  GEOLOGY 


Fia.  12. — Inclined  fold  Temescal  Ranch,  Ventura  County. 
(After  Watts,  Calif.  Mining  Bureau,  Bull.,  19.) 


FIG.  13. — View  of  South  Mountain  anticline  near  Santa  Paula,  Calif. 


STRUCTURAL  GEOLOGY 


53 


When  the  basins  are  filled  with  water,  oil  may  be  found  on  the 
flanks  of  synclines.  When  little  or  no  water  occurs  in  the  basin, 
oil  may  be  found  close  at  the  bottom  of  the  depression.  (Figs. 
15,  16  and  17.) 


FIG.  14. — Steeple-shaped  anticline  overturned  at  top. 


FIG.   15. — Illustrates  possibility  of  finding  oil  in  synclines. 


FIG.  16. — Illustrates  an  asymmetrical  syncline. 

Deformation. — The  amount  of  arching  is  called  the  deformation 
of  the  structure.  It  is  measured  from  the  bottom  of  the  syncline 
to  the  top  of  the  anticline.  (See  Fig.  18.) 


54 


PRACTICAL  OIL  GEOLOGY 


In  California,  Wyoming,  and  other  steeply  folded  areas, 
deformations  of  500  to  5000  ft.  are  known. 

In  Oklahoma  and  Kansas  deformations  of  10  to  200  ft.  are 
known,  but  by  far  the  larger  proportion  of  structures  show  only 
40  to  60  ft.  deformation.  Gushing,  the  most  productive  of  all 


Dry  Hole 


FIG,  17,— Illustrates  complex  folding.     (After  U,  S.  G-  S.) 


Oklahoma  pools,  shows  160  ft.  from  the  syncline  to  the  top  of 
the  dome.  Such  low  deformations  usually  occur  in  distances  of 
half  a  mile  to  a  mile.  An  East  dip  160  ft.  in  a  distance  of  6 
miles  occurs  near  Onaga  in  Pottawatomie  County,  Kansas. 


KANSAS-OKLAHOMA-TEXAS 
FIG.  18. 

3.  Monoclines.— A  monocline  is  a  structure  with  one  slope  or 
inclination.  Its  name  comes  from  mono,  one,  and  clino,  sloping. 

Monoclines  are  simple  structures  as  shown  in  Fig.  19.  They 
are  often  limbs  or  flanks  of  giant  anticlinal  folds  or  of  giant  domes, 
where  but  one  side  of  the  fold  is  apparent  and  that  dipping  in  one 
direction.  The  northeastern  Oklahoma  oil-fields  are  located 


STRUCTURAL  GEOLOGY 


55 


on  minor  folds  that  occur  on  a  great  northwestern  dipping 
monocline. 

Normal  Dips. — In  all  regions  of  uplift  the  dip  of  the  beds  is 
naturally  away  from  the  center  or  line  of  the  uplift.     The  great 


X    X  X  X 
<.  X    XX   X 
X  X   X  X 
C  X  XX  X 
X   X  X  X 
XXXXXXXX   XXXXXXXXXXXXXXXX 

Xxxxxxxxxxxxxxxxxxxxxxxxxxx 


XXXX  XXXX 

<xxxxxxxx 

XXXXXXXXX 
< xxxxxxxxxXXXXXXXX 


FIG.  19. — Illustrates  wells  on  a  monocline. 

monocline  formed  by  these  dipping  beds  are  broken  by  minor 
folds.  The  dip  of  the  monocline  as  a  whole  is  called  the  normal 
dip  of  the  country,  and  decreases  in  intensity  from  the  center  of 
folding  outward.  In  Kansas  and  northern  Oklahoma  the  normal 
westward  dips  vary  from  15  to  30  ft.  per  mile.  In  central 


Gas 


FIG.  20a. — Cross-section  of  a  terrace. 


and  southern  Oklahoma  the  normal  dips  vary  from  40  to  70  ft. 
per  mile.  In  central  Texas  40  to  70  ft.  per  mile  is  general; 
while  in  Mississippi  and  Louisiana  20  to  30  ft.  per  mile  are 
average  dips. 

To  obtain  an  accumulation  of  oil  on  these  monoclines  it  is 


56  PRACTICAL  OIL  GEOLOGY 

essential  to  find  a  break  or  check  in  this  normal  dip.  Accord- 
ingly, if  a  reversal  or  flattening  of  dip  is  found,  an  accumulation 
of  oil  may  take  place  if  reservoir  conditions  are  favorable, 
and  material  for  the  formation  of  oil  occurred  in  the  region. 

Terrace  structure  is  practically  monoclinal  as  the  major  dip  is 
in  one  direction.  The  famous  Glen  Pool  of  Oklahoma  is  on  such 
a  structure.  Terrace  structure  is  a  combination  of  an  anticline 
and  a  syncline  for  with  such  structures  the  fold  has  not  been 


FIG.  206. — Contour  map  of  a  terrace. 

completed  to  the  point  where  a  well-defined  reversal  of  dip  has 
developed.  The  flat  terrace  is  shown  in  Fig.  20a  by  a  cross- 
section,  and  in  Fig.  206  by  contour  lines. 

Terraces. — A  terrace  may  be  favorable  for  oil  accumulation 
in  shallow  regions  where  water  pressures  are  low,  but  tests  in 
Oklahoma  under  favorable  conditions  show  that  sands  on  terraces 
when  found  at  depths  over  2000  ft.  have  proven  small  producers. 
Theoretically  there  is  a  good  reason  for  this  small  production, 
but  in  this  book  the  writer  does  not  have  time  to  go  into  a 
mathematical  discussion.  Suffice  it  to  say  that  if  the  friction 
or  adhesion  of  the  oil  in  the  sand,  and  also  the  cohesion  of  the 
particles  of  oil  is  sufficient  to  overcome  the  difference  in  specific 
gravity  of  water  and  also  the  force  of  moving  water,  then  ac- 
cumulation may  take  place. 


STRUCTURAL  GEOLOGY 


57 


4.  Combinations  of  Monoclines,  Synclines,  and  Anticlines. — 

The  combination  of  a  monocline,  a  syncline,  and  an  anticline 
into  one  structure  is  a  very  common  occurrence.  Such  fields  are 
well  illustrated  by  the  Coalinga  and  the  Simi  valley  oil  fields  of 
California,  where  one  finds  barren  igneous,  or  metamorphic 
measures  on  one  side,  and  a  series  of  folds  in  the  sedimentaries 
trending  away  from  the  igneous  or  metamorphic  rocks.  Fig. 
21  illustrates  a  monocline  immediately  on  the  flank  of  the  igne- 
ous or  metamorphic  rocks,  then  a  syncline  and  then  an  anti- 
cline, or  a  series  of  them. 


Monocline 


Syncline 


Anticline 


FIG.  21. — Illustrates  a  monocline,  syncline,  and  anticline  combined.     Note 
unconformity,  also  splitting  of  sands.     (After  U.  S.  G.  S.) 

5.  Domes. — A  dome  or  quaquaversal  is  a  structure  in  which  the 
strata  dip  from  a  central  point  rather  than  from  an  axis  or  line. 
Domes  are  circular  or  elliptical  and  are  divided  into  three  main 
classes:  (a)  Anticlinal  domes,  (6)  volcanic  domes,  and  (c)  saline 
domes. 

ANTICLINAL  DOMES. — Anticlinal  domes  are  those  high  points 
or  crests  along  the  top  of  undulating  anticlines.  Such  forms  of 
domes  are  very  common  in  California,  Oklahoma,  Wyoming, 
Pennsylvania,  and  throughout  the  oil  fields  of  India.  In  many 
places  one  main  anticlinal  fold  may  continue  for  10  to  60 
miles  and  undulate  along  its  course,  forming  many  anticlinal 
domes  or  quaquaversals  that  localize  the  accumulation  of  oil. 
(See  Fig.  22a).  In  some  cases  anticlines  intersect  as  is  shown 
in  Fig.  226,  and  in  other  cases  anticlines  merge  into  one  another 
as  in  Fig.  22c.  The  resulting  structure  in  each  case  is  generally 
an  anticlinal  dome. 


58  PRACTICAL  OIL  GEOLOGY 

Where  such  a  quaquaversal  structure  stands  alone  it  is  simply 
called  a  dome.  A  knowledge  of  these  domes  is  essential  to  in- 
telligent prospecting  as  they  make  oil  territory  " spotted." 


FIG.  22. — Illustrates  by  contours  various  forms  of  doming  on  anticlines, 
a,  domes  on  major  anticline,  &,  dome  formed  by  two  cross  anticlines;  c, 
dome  formed  by  merging  anticline. 

Realization  of  this  condition  will  save  a  great  deal  of  money 

to  oil  operators  who  appreciate  the  value  of  structural  geology. 

As  shown  in  Fig.  44,  Chapter  V,   the  dips  indicated  by  the 


STRUCTURAL  GEOLOGY 


59 


little  darts  form  an  elliptical  structure.  The  plunges  of  the 
anticline  are  shown  by  the  arrows  on  its  axis.  The  plunges  along 
the  axes  of  the  anticline  are  generally  less  than  the  dips  away 
from  the  axes.  Where  the  plunges  are  equal  to  the  dips,  the 
domes  are  circular. 

IMPORTANCE  IN  RELATION  TO  PETROLEUM. — Dome  struc- 
ture is  the  most  favorable  for  the  accumulation  of  petroleum, 
as  the  oil  rises  from  a  large  area  to  the  apex  of  the  dome. 


Oil  Well 


Igneous  Neck 


Oil  Well 


FIG.  23. — Illustrates  occurrence  of  oil  around  Volcanic  neck  in  Mexico. 

The  concentration  of  the  oil  must  necessarily  be  localized  as 
the  tendency  of  the  petroleum,  where  the  oil  strata  are  satu- 
rated with  water,  is  to  rise  to  the  top  of  the  dome,  just  as  in  an  anti- 
cline the  oil  rises  to  an  axis.  As  a  result  of  doming,  however, 
the  oil  is  concentrated  into  a  large  reservoir  around  a  central 
point  on  the  axis  instead  of  being  concentrated  along  the  line  of 
the  anticlinal  axis. 

VOLCANIC  DOMES. — Volcanic  domes  are  those  formed  by  the 
intrusion  of  volcanic  matter  into  and  through  sedimentaries  to 
form  structures  like  that  shown  in  Fig.  23.  The  formations 


60 


PRACTICAL  OIL  GEOWGY 


around  the  cores  or  necks  are  bent  upward  forming  domes  that 
act  as  reservoirs  for  the  accumulation  of  petroleum.  The  pres- 
ence of  igneous  rocks  is  by  no  means  to  be  considered  as  detri- 
mental to  the  presence  of  petroleum  where  such  rocks  are  in- 
trusives.  These  volcanic  necks  are  found  particularly  in  the 
Mexican  oil  fields. 

SALINE  DOMES. — Saline  domes  have  cores  of  salt  at  their 
centers.  It  is  thought  by  some  that  underground  waters  brought 
great  quantities  of  salt  upward  through  a  fault  or  fissure,  and  the 
recrystallization  of  the  salt  resulted  in  so  great  an  expansion  that 
the  upward  bowing  of  the  formations  overlying  the  salt  beds 
formed  domes.  (See  Fig.  24.)  The  more  plausible  theory  is  that 


FIG.  24. — Illustrates  saline  dome.     Oil  occurs  in  the  dolomite. 
(After  Lee  Hager.) 

the  salt  core  is  but  part  of  a  deep  lying  salt  stratum,  which  has 
been  sharply  folded  and  faulted,  and  the  salt  bed  encountered  in 
drilling  into  the  heart  of  the  dome  is  that  part  of  the  bed  which  has 
been  forced  up  along  the  fault  plane.  Such  saline  domes  are 
common  in  Texas  and  Louisiana. 

6.  Faulted  Forms. — Faults  are  planes  of  rupture  in  rocks  due 
to  the  slipping  or  sinking  of  strata  upon  one  another  caused  by 
earth  movements.  Faults  are  of  many  kinds,  but  a  few  sketches 
will  make  plain  how  they  affect  oil  sands,  and  why  a  knowledge 


STRUCTURAL  GEOLOGY 


61 


of  them  is  important.  Faults  are  closely  related  to  folds  and 
in  most  places  are  indeed  but  broken  folds.  Faults  at  the  sur- 
face often  change  to  folds  underground. 


Angle  of 
Dip   \ 


Horizontal 
Displacemen 
("  Heave  ") 


FIG.  25. — A  normal  fault. 

There  are  two  main  classes  of  faults:  (1)  Normal  (see  Fig.  25) 
and  (2)  Reserved  (see  Fig.  26).  The  names  of  the  different  parts 
of  faults  afe  given  on  the  figures. 


FIG.  26. — A  reverse  fault. 

Where  a  fault  occurs  in  a  field,  oil  deeply  buried  may  often 
migrate  along  the  fault  plant  to  beds  higher  up  and  thus  enable 
one  to  procure  production  at  a  shallower  depth  than  expected. 


62 


PRACTICAL  OIL  GEOLOGY 


Sometimes  great  masses  of  brea  or  asphalt  occur  along  fault 
lines  and  extend  for  a  number  of  miles.  In  the  upper  Ojai 
valley  of  California,  and  at  Santa  Maria,  California,  such  is 
notably  the  case 


Fault 


FIG.  27. — Illustrates  faulting.     Note  unbroken  anticline. 

Effect  of  Faulting. — The  upthrow  side  of  a  fault  acts  like  the 
higher  part  of  an  anticline  and  is,  in  consequence,  the  most 
favorable  place  on  a  faulted  structure  to  drill  for  oil. 

In  Mexico  the  largest  oil  accumulations  occur  along  fault 
planes.  Where  a  major  fault  is  intersected  by  a  minor  fault 


FIG.  28. — Shows  effect  of  faulting.     Dry  hole  to  right  of  fault. 
(After  U.  S.  G.  S.) 

the  greatest  accumulation  occurs  on  the  upthrow  side  of  the 
major  and  minor  faults. 

However,  where  faulting  is  too  severe,  it  may  throw  the  oil 
formations  very  deep  on  the  downthrow  side. 

In  a  normal  fault,  if  the  vertical  displacement  or  throw  is 
great,  the  depth  to  oil  may  be  very  materially  affected  by  the 


STRUCTURAL  GEOLOGY 


63 


break.  Reversed  faulting  is  most  frequent  in  oil-field  work,  and 
is  due  to  the  pushing  of  one  series  of  strata  over  another.  Re- 
versed faulting  more  often  occurs  on  anticlines,  normal  faults  in 
synclines. 

Another  classification  is  dip  fault  and  strike  fault.  Dip 
faults  are  in  the  direction  of  the  dip;  strike  faults  are  parallel 
to  the  strike  of  the  strata.  Most  productive  fields  are  cut 
by  faults  so  that  they  are  common  features.  A  few  examples 
of  faulted  structures  are  shown  in  Figs.  27,  28,  29a  and  296. 


FIG.  29  a.— Illustrates  thrust  fault. 


FIG.  296. — Illustrates     disappear- 
ance of  fault  with  depth. 


It  musf  be  clearly  borne  in  mind  that  there  are  a  large  number 
of  different  forms  of  structures.  Those  presented  above  form 
only  the  chief  folds  that  are  important  for  oil  men  to  consider. 

Not  all  Folds  are  Productive  in  Oil  Regions. — Most  oil  fields 
occur  on  folds,  but  there  are  cases  where  the  folds  themselves 
may  be  well  pronounced  and  in  oil  bearing  regions,  but  conditions 
are  not  favorable  for  accumulation  of  oil. 

Unfavorable  conditions  may  be  briefly  classified  as  follows: 

1.  Lack    of    sands,    sandstones    or    limestones    favorable    as 
reservoirs.     Occur  in  nearly  all  oil  regions. 

2.  Sand  or  sandstones  or  limestones  present  but  too  closely 
cemented  or  "tight"  to  hold  oil.     Occur  in  Mid-Continent  and 
Eastern  fields. 

3.  Porous  formations  with  insufficient  water  in  the  sands  to 
force  the  oil  in  the  formations  to  the  top  of  the  folds.     Oil  in 
such  cases  will  lie  on  the  sides  of  the  folds  close  to  the  syncline. 


64 


PRACTICAL  OIL  GEOLOGY 


Such  is  notably  the  case  in  some  Kentucky  and  Pennsylvanian 
fields. 

4.  Presence  of  intrusives  or  old  land  masses  at  the  center  of 
the  fold.     An  intrusive  in  Mexico  is  illustrated  by  Fig.   30. 
Wells  Nos.  1  and  3  strike  oil;  well  No.  2  was  drilled  into  granitics. 
At  Zeandale  and  at  Elmdale,  Kansas,  the  expected  oil  horizons 
were  not  found,  but  horizons  thought  to  be  considerably  older  are 
far  nearer  the  surface  than  they  should  normally  occur  indicating 
an  old  land  mass.1 

5.  Erosional  or  angular  unconformities  may  affect  the  accumu- 
lation of  oil  as  shown  in  Figs.  31  and  32. 


FIG.  30. 


FIG.  31. 


FIG.  32. 


In  the  erosional  unconformity  (Fig.  31)  wells  Nos.  1  and  4  are 
productive;  Nos.  2  and  3  are  barren.  Such  a  condition  is  due  to 
the  erosion  or  washing  of  the  earlier  beds  before  the  later  beds 
were  deposited  upon  them,  leaving  an  island-like  condition. 

Angular  unconformities  (see  Fig.  32)  may  result  in  a  lack  of 
production  in, a  fold.  Well  No.  1  is  barren;  No.  2  is  productive. 
Were  No.  1  drilled  first,  the  field  would  perhaps  be  abandoned. 
Were  No.  2  drilled  first,  later  developments  at  No.  1  would  prove 
disastrous.  Such  a  condition  is  the  result  of  folding  or  tilting  of 
the  beds  prior  to  the  later  deposition  and  folding. 

6.  Faulting  may  affect  accumulations,  but  that  has  been  dis- 
cussed earlier  on  pages  61  and  62. 

Folds  Productive  in  Spots. — Folds  not  productive  at  one  place 
often  show  good  wells  at  others.  This  is  due  to  sand  conditions. 

1  Granite  has  been  definitely  proven  in  the  Onaga,  Kansas  area  bearing 
out  the  results  at  Zeandale. 


STRUCTURAL  GEOLOGY 


65 


Porous  sands  may  not  lie  as  blankets  over  a  fold,  but  occur  in 
spots  on  the  structure.  Fig.  33  clearly  illustrates  this  condition. 
A  well-defined  structure  is  shown  but  it  is  productive  in  spots. 
Gas  occurs  at  the  top  of  the  spots,  the  oil  occurs  lower,  and 
water  below  the  oil. 

One  dry  hole  on  such  a  structure  would  by  no  means  condemn  it. 


^  Gas  Well 
•  Oil  Well 
6}  Dry  Hole 


FIG.  33. 

STRATIGBAPHIC   FORMS — CONDITIONS  WHERE  STRUCTURE  DOES 

NOT  GOVERN 

Favorable  Reservoirs  of  Much  Importance. — Structure  is 
undoubtedly  extremely  important  in  petroleum  accumulations, 
but  emphasis  must  also  be  placed  on  sand  conditions.  Without 
favorable  sand  conditions  and  sufficient  material  for  the  formation 
of  oil,  structure  is  worthless.  Again,  in  special  cases  oil  may  ac- 
cumulate where  definite  geologic  structure  is  not  known,  but 
favorable  reservoirs  have  been  formed  in  the  sands,  as  explained 
below. 


66 


PRACTICAL  OIL  GEOLOGY 


Lenses. — Spots. — Accumulations  where  structures  are  not 
apparent  occur  in  numerous  places.  The  peculiar  relation  is  well 
illustrated  in  Fig.  34.  Where  the  normal  dip  of  the  region  is  in 
one  direction  and  a  body  of  sand  gradually  pinches  out  or  lenses 
up  the  dip  as  in  Fig.  35  there  is  an  ideal  condition  for  accu- 


FIG.  34. 

mulation  of  oil  or  gas.  Surface  indications  show  no  presence  of 
structure,  but  an  actual  test  may  show  oil.  A  number  of  the 
Oklahoma  "pools"  occur  from  such  causes,  notably  pools  in  the 
Broken  Arrow,  Morris,  and  Okmulgee  districts,  where  sand 
changes  are  very  rapid. 


FIG.  35. 

Again,  lensing  of  the  sand  may  be  due  to  the  playing  out  of 
the  sand  in  two  directions,  as  in  Fig.  35.  In  either  case  condi- 
tions favorable  for  accumulation  occur.  These  sand  changes 
occurred  when  the  sand  was  first  deposited.  Similar  conditions 
may  be  noted  along  any  lake  front  or  sea  coast. 


STRUCTURAL  GEOLOGY 


67 


Productive  spots  due  not  to  the  thickening  of  the  sand,  but 
resulting  from  porous  spots  in  tight  or  closely  cemented  sands 
are  of  common  occurrence.  The  cementation  or  filling  of  the 
pore  space  between  the  sands  with  siliceous  or  lime  salts  cause 
conditions  of  accumulation  similar  to  those  caused  by  lensing. 

Changes  in  Character  of  Sands. — Oil  sands  in  themselves 
vary  in  thickness  even  in  the  same  field.  A  rough  sketch  illus- 
trates how  a  sand  may  change.  (See  Fig.  35.)  At  No.  1  there 
are  50  ft.  of  sand;  at  No.  2,  there  are  20  ft.  of  sand;  at  No.  3 
the  sand  has  divided  or  has  been  split  by  shale;  at  No.  6  the 
upper  sand  has  changed  entirely  to  shale. 


FIG.  36. — Sand  map. 

Sand  Maps. — Wherever  possible,  sand  maps  should  be  con- 
structed. Such  maps  are  explained  by  contours,  but  the  contour 
lines  here  stand  for  equal  thickness  of  sand  and  not  structure 
contours.  (See  Fig.  36.) 

Developing  Lenses. — The  geologist  can  be  of  material  assist- 
ance in  aiding  the  operator  in  developing  lenses. 


68  PRACTICAL  OIL  GEOLOGY 

Suppose  a  well  is  "brought  in"  and  upon  examination  a  normal 
dip  condition  is  found,  no  structure  in  other  words,  then  the 
geologist  has  the  following  basis  to  go  on; 

Where  surface  beds  show  lack  of  folding,  then  lensing  and  spots 
are  the  most  probable  explanations,  though  unconformities  or 
hidden  faults  may  complicate  matters.  The  history  of  the  oil- 
fields, if  in  an  old  producing  area,  will  serve  to  guide  the  engineer 
as  to  whether  or  not  lensing  is  probable. 

The  history  of  some  pools  show  the  average  direction  of  the 
productive  trend  of  lenses  and  spots  to  be  parallel  to  the  old 
shore  lines;  in  Oklahoma  and  Kansas  this  is  in  a  northeasterly 
direction. 

Lenses  and  "spots"  may  change  very  rapidly  so  it  is  advisable 
to  keep  the  wells  close  together,  and  develop  the  "pool"  by 
starting  the  new  tests  in  close  proximity  to  the  producing  wells, 
thus  avoiding  superfluous  "dry"  holes. 

The  presence  of  water  below  the  oil,  or  gas  above  it,  in  the 
same  sand,  the  dip  of  the  surface  rocks,  and  the  thicknesses  of 
the  gas,  oil  and  water  impregnated  portions  of  the  sand,  aid  in 
approximating  the  "up-dip"  or  "down-dip"  limits  of  the  pool. 

The  gravity  of  the  oil,  if  higher  than  in  nearby  pools,  would 
suggest  a  relatively  small  pool  of  migratory  oil. 


CHAPTER  V 
PROSPECTING  AND  MAPPING 

In  searching  for  petroleum  fields,  bear  in  mind  the  following 
points: 

1.  Oil  occurs  in  sedimentary  rocks  of  marine,  estuarine  and 
deltaic  origin,  i.e.,  beds  formed  along  sea  shores,  in  inland  bays, 
at  river  mouths,  etc. 

2.  Igneous    rocks    do    not    contain    oil.     Several    exceptions 
to  this  rule  may  be  cited,  but,  wherever  investigated,  the  oil  was 
found  in  conjunction  with  sedimentaries. 

3.  Oil  is  almost  always  associated  with  some  form  of  a  folded 
or  arched  structure.     (Anticlines,  monoclines,  terraces,  and  domes 
are  favorable  structures  for  the  accumulation  of  oil.     Synclines 
in  some  cases  contain  oil.) 

4.  Petroleum  seepages,  burned-out  shales,  oil-stained   sands 
and  shales, _"  gas  burns,"  gas  bubbles  on  water,  and  asphalt  or 
brea  beds  are  all  indicators  of  petroleum. 

By  bearing  the  above  points  in  mind,  one  can  enter  a  new 
field  and  quickly  determine  whether  or  not  it  is  favorable  for  oil. 

One  first  concentrates  attention  on  the  sedimentary  strata. 
If  igneous  formations  are  known,  eliminate  such  regions  from 
consideration  except  where  volcanic  necks  or  plugs  lift  the 
formations  to  form  favorable  structures  for  the  accumulation 
of  oil,  as  occur  in  Mexico  and  probably  Texas. 

If  possible,  determine  whether  or  not  the  oil  formations  are 
of  marine,  estuarine,  or  deltaic  origin.  Recent  lake  beds  and 
marshes  do  not  carry  oil  enough  to  form  commercial  deposits. 
Sandstones,  sands,  shales,  and  coarse  dolomitic  limestones 
should  be  sought  as  favorable  reservoirs  for  oil. 

Next  look  for  structural  conditions  favorable  to  the  accu- 
mulation of  oil.  Such  structures  are  generally  folded  or  faultec^ 
with  sufficient  inclination  to  the  strata  to  allow  oil  to  rise  above 

69 


70  PRACTICAL  OIL  GEOLOGY 

the  water  and  accumulate  at  the  top  .of  the  structures.  Any  of 
the  favorable  types  of  structure  shown  in  Chapter  IV  afford 
places. for  the  accumulation  of  petroleum. 

In  some  regions,  notably  east  Texas  and  western  Louisiana, 
structural  conditions  are  difficult  to  determine,  due  to  the  flat- 
ness of  the  beds  and  the  effect  of  erosion,  which  has  made  the 
topography  nearly  flat. 

Oil  signs  such  as  seepages,  brea  beds,  and  "gas  shows"  may 
or  may  not  occur  in  a  region  that  contains  oil  in  commercial 
quantities.  If  the  oil-bearing  formations  are  thickly  covered  with 
alluvium  or  other  non-productive  beds,  oil  may  not  appear  at 
the  surface  though  the  structure  may  be  favorable  to  the  accumu- 
lation of  oil.  One  may,  however,  find  oil  in  springs,  or  staining 
the  small  streams  that  cut  through  oil  sands.  Oil-stained  rock 
and  "gas  shows"  are  favorable  signs. 

Light  gravity  oils  do  not  form  heavy  asphalt  beds,  as  the 
constituents  are  readily  washed  away  by  water. 

The  best  places  to  look  for  evidences  of  oil  are  in  the  beds  of 
gullies  and  canons,  and  on  the  steep  erosion-scarps  or  cliffs 
that  occur  in  many  regions.  Old  mines  and  water  wells  are  all 
good  places  to  search  for  oil  and  gas  indications,  especially  where 
oil  rises  on  top  the  water. 

Cautions. — Investigators  have  many  times  been  called  to 
regions  where  iron  and  manganese  oxides,  black  alkali,  or 
vegetable  stain  was  thought  to  be  oil.  Sometimes  gas  escaping 
from  springs  gives  clues  to  petroleum.  Be  sure,  however, 
that  the  gas  will  burn  as  many  non-hydrocarbon  gases,  such 
as  sulphur  dioxide  and  carbon  dioxide,  are  non-combustible. 
Marsh  gas  from  old  lake  beds  has  many  times  been  considered 
a  good  sign  of  petroleum  and  led  to  oil  excitement,  but  marsh 
gas  occurs  in  many  regions  where  petroleum  does  not  exist. 

Sometimes  oil  sands  are  almost  white,  due  to  sulphur  stains, 
and  one  would  scarcely  suspect  them  to  contain  petroleum.  Dig 
into  such  beds  a  few  inches  and  chocolate,  greenish,  brown  or 
black-colored  oil  sand  may  appear. 


PROSPECTING  AND  MAPPING  71 

An  interesting  and  valuable  test  for  oil  is  given  below: 

TEST  FOR  OIL  IN  ROCKS 

(After  E.  G.  Woodruff) 

1.  Select  a  representative  specimen  of  rock  to  be  tested.    To  secure  a 
representative  sample,  it  is  generally  advisable  to  secure  several 
samples  as  large  as  one  to  five  pounds. 

2.  Break  them  up,  thoroughly  mix  the  pieces.     If  sand,  mix  the 
sand. 

3.  Dry  the  sample  in  the  sun  or  over  a  radiator.     Do  not  dry  it 
over  a  fire;  to  do  so  may  drive  the  oil  from  the  rock  or  sand. 

4.  Crush  the  sample  to  a  powder.     Mix  the  powder.    Loose  sand 
does  not  need  to  be  crushed. 

5.  Place  about  a  tablespoonful  of  the  sample  in  a  bottle.     Pour 
chloroform  or  carbon-tetra-chloride  over  the  sample  until  it  is 
thoroughly  saturated  and  there  is  about  a  half  tablespoonful  of 
the  liquid  above  the  crushed  rock  or  sand.    Cork  the  bottle,  but 
not  too  tightly.     Shake  occasionally  for  fifteen  or  twenty  minutes. 

6.  Place  a  white  filter  paper  in  a  glass  funnel  with  a  white  enamel 
dish  below  the  funnel. 

7.  Pour  the  contents  of  the  bottle  into  the  funnel.     After  the  liquid 
has  filtered  through,  place  the  white  dish  in  a  window  where  the 
liquid  can  evaporate. 

8.  Examine  the  filter  paper.     If  the  rock  contains  more  than  a  trace, 
there  will  be  a  brown  or  black  ring  on  the  filter  paper. 

9.  After  the  liquid  in  the  dish  has  evaporated,  examine  the  remain- 
ing substance.     It  is  the  petroleum  which  was  in  the  rock. 
Apparatus  needed: 

One  dinner  plate  upon  which  to  dry  specimens. 

Some  means  for  crushing  rock. 

One  or  more  bottles,  4  or  6  oz.  size  (with  corks)  in  which  to  treat 

the  rock. 

Chloroform  or  carbon-tetra-chloride. 
One  glass  funnel  three  or  four  inches  in  diameter. 
Two  dozen  round  filter  papers,  six  inches  in  diameter. 
Two  or  more  white  dishes,  the  small  serving  dishes,  such  as  are 

used  at  hotels,  are  good;  they  can  be  purchased  at  a  ten-cent 

store. 


72 


After  the  investigator  is  fully  satisfied  that  a  region  is  worth 
investigating  carefully,  the  hard  work  begins.  Detailed  struc- 
ture is  mapped  and  a  study  made  as  to  the  various  types  and 
forms  of  structure.  A  geological  map  is  constructed  and  detailed 
cross-sections  worked  out.  In  making  such  a  map  and  cross- 
sections,  one  resorts  to  the  methods  and  symbols  shown  in  this 
chapter  under  "Mapping."  Such  work  carefully  done  enables 
one  to  determine  the  following  points  : 

1.  The  places  worth  testing  with  a  drill,  i.e.,  the  anticlines, 
terraces,  monoclines,  or  domes. 

2.  The    presence    of   faults,    synclines,    and    intrusives    that 
may  adversely  or  favorably  affect  the  accumulation  of  petroleum. 
As  noted  before,  faults,  synclines  and  intrusives  do  not  neces- 
sarily adversely  affect  a  field,  but  they  sometimes  do. 

3.  The  relative  ages,  thicknesses,  and  character  of  the  forma- 
tions of  the  district.     Such  information  is  very  valuable  when 
extensions  of  an  oil  field  are  considered.     Oil  found  at  certain 
horizons  in  one  district  does  not  necessarily  occur  at  the  same 
horizon  in  other  districts,  although  it  often  does. 

4.  Length  and  breadth  of  the  field. 

5.  The  accessibility  of  the  region,  road  conditions,  fuel  supply, 
water,  and  ease  of  securing  supplies  are  also  factors  that  must 
be  seriously  considered  before  actual  drilling  commences.     Some 
districts  are  geologically  favorable  for  oil,  but  when  haulage  of 
drilling  materials  costs  $20  to  $30  per  ton  it  will  not  pay  to 
prospect  with  a  drill. 

Prospecting  by  means  of  crooked  sticks,  magnetic  instru- 
ments, and  electrical  contrivances  is  sometimes  employed. 
Where  the  examining  man  is  an  expert  oil  man,  he  may  determine 
favorable  oil  territory  despite  his  instruments,  but  generally  such 
men  are  "  fakers,"  pure  and  simple. 

Before  actually  starting  drilling  in  a  new  country,  spend  some 
time  prospecting.  More  money  can  often  be  saved  by  a  few  days 
spent  in  faithful  prospecting  than  by  spending  $50,000  to  put 
down  a  drill  hole. 

The  appended  table  taken  from  Cunningham  Craig's  "Oil 
Finding"  may  be  of  service. 


PROSPECTING  AND  MAPPING 


73 


TABLE  IX. — FAVORABLE  AND  UNFAVORABLE  INDICATIONS  OF  OILS 


Favorable 


Unfavorable 


Always 

Usually 

Sometimes 

Usually 

Always 

Shows    of     oil 

Shows  of 

Shows   of   oil 

Light  shows  of 

with  strong  gas 

filtered  oil 

with  very 

oil   in  thick 

in  thin  porous 

with  gas. 

little  gas. 

porous  beds 

beds  along  im- 

with   water 

pervious  strata 

or  brine. 

Evidence  of 

Evidence    o  f 

estuarine     o  r 

entirely  ma- 

deltaic condi- 

rine    condi- 

tions. 

tions. 

Beds  of  gyp- 

sum or  rock 

salt. 

Shows    of    gas 

Brine. 

below  or  in  a 

thick    argilla- 

ceous series. 

Shows  of  par- 

Shows of  par- 

tially     inspis- 

tially in- 

_- 

sated  oil  near 

spissated  oil 

the  surface. 

deep  down. 

Water-sands 

below    a 

thick  argil- 

laceous ser- 

ies. 

Lignites  or 

Sulphuretted 

Hot    water 

coal,  fossil 

hydrogen 

with  neither 

resin,     s  u  1  - 

accompa- 

oil nor  gas. 

phur  or  sul- 

nied      by 

phure  tted 

hot  water. 

hydrogen. 

Gas  in  slightly 

Gas  shows 

porous  strata 

accompan- 

with pressure 

ied  by  water 

increasing 

in  porous 

downward. 

beds  among 

impervious 

beds. 

Ozokerite  or 

manjak  veins. 

74  PRACTICAL  OIL  GEOLOGY 

MAPPING 

Before  proceeding  further  with  a  detailed  description  of  oil-field 
structure  it  is  well  to  study  the  graphic  representation  of  typical 
structures  by  means  of  topographic  and  of  geologic  symbols. 
The  United  States  Geological  Survey  has  made  excellent  geo- 
logical maps  of  most  of  the  American  oil-fields,  and  for  that 
reason  the  oil  men  should  learn  to  interpret  the  maps  to  best 
advantage. 

Topography. — Topography  is  the  description  of  the  earth's  sur- 
face and  the  study  of  the  surface  forms,  hills,  valleys,  drainage, 
etc.  Fig.  37  illustrates  a  method  of  expressing  surface  features  by 
a  topographic  map.  The  hills  are  shown  by  means  of  contour 
lines,  which  are  lines  connecting  points  of  equal  elevations  above 
mean  sea  level,  the  lines  being  drawn  at  regular  vertical  intervals. 
The  numbering  on  the  lines  indicates  the  heights  above  mean 
sea  level. 

A  skilled  reading  of  topographic  sheets  often  enables  one  to 
point  out  anticlines,  synclines,  and  faults  without  even  knowing 
the  district. 

It  is  more  important  for  the  oil-man  to  study  sedimentary 
topography  than  to  study  igneous  topography.  The  following 
brief  summary  may  be  of  value: 

Granitic  and  volcanic  rocks  give  a  rugged  broken  surface. 
Shales  and  soft  sandstone  erode  to  form  rounded  hills  and  wide 
valleys.  Hard  sandstone,  limestone,  and  conglomerates  form 
steep-faced  hills  and  escarpments  when  the  dips  are  moderate; 
when  the  beds  dip  steeply,  erosion  will  in  time  wear  down  the  soft 
shales  and  sandstones  to  a  bed  of  limestone  or  sandstone  that  will 
not  readily  wear  away.  *  In  such  cases  where  the  beds  have  been 
folded  the  limestone  may  occur  higher  in  the  hills  than  in  the 
valleys,  a  case  where  a  " topographic  high"  is  a  "structural 
high." 

Hard  beds  erode  slowly  and  soft  ones  rapidly,  giving  a  short 
bench-like  form  to  the  hills,  where  hard  beds  are  present.  Ter- 
races of  this  kind  are  very  common.  By  watching  such  terraces 


PROSPECTING  AND  MAPPING 


75 


one  can  trace  beds  over  considerable  distances,  even  though  rock 
exposures  are  not  clearly  visible  or  are  covered  with  vegetation. 
Hills  capped  with  hard  limestone  or  sandstone  beds  often  reflect 
the  dips  very  accurately,  so  that  one  can  see  the  slope  with  the 
eye. 


FIG.  37. — Ideal  sketch  and  corresponding  contour  map. 
(After  U.  S.  G.  S.) 


Drainage. — In  studying  probable  prospective  territory  it  is 
often  of  service  to  study  the  drainage  lines.  The  main  streams  in 
many  regions  cut  across  the  major  folds,  and  the  smaller  and 
secondary  streams,  follow  the  major  structure,  such  as  synclines, 
anticlines  and  faults. 

In  some  places  the  main  stream  follows  the  synclines  and  crosses 
the  main  anticlinal  folds  at  the  saddles  or  low  places  on  the  anti- 
cline. High  divides  often  point  to  anticlinal  conditions.  No  rule 


76 


PRACTICAL  OIL  GEOLOGY 


can  be  stated,  however,  to  cover  all  regions,  so  each  region  must 
be  considered  as  a  distinct  topographic  unit. 

Underground  structure  is  generally  reflected  by  the  surface. 
This  truth  must  not  be  forgotten.  Hills  and  mountains  often 
reflect  underground  conditions  so  that  wherever  one  sees  hills 
he  naturally  looks  for  some  sort  of  structural  change,  although 
this  is  not  generally  the  case. 

Valleys  are  many  times  synclinal.  Also  the  crests  of  many 
anticlines  have  been  eroded  or  washed  out  to  form  valleys,  and 
the  apices  of  domes  are  eroded  in  the  same  way.  In  such  cases, 
while  the  flanks  of  the  anticlines  may  be  high,  the  centers  are  cut 
by  valleys.  Faults  may  also  cause  valleys  by  leaving  depres- 
sions, or  subjecting  some  parts  of  the  structures  to  more  rapid 
erosion  than  other  parts. 

Dip  Slopes. — The  dips  of  the  beds  in  Kansas,  Oklahoma,  and 
the  Pennsylvania  area  of  Texas  are  generally  reflected  in  the 
table-like  tops  of  the  hills  which  are  capped  with  sandstones  or 
limestones,  that  accurately  delineate  the  dip  of  the  beds.  This 
fact  has  proven  of  such  great  value  to  the  geologist  that  it  has 
enabled  him  to  work  country  in  several  days  time,  that  would 
otherwise  require  several  weeks. 

Erosion  Features. — Erosion  in  synclines  may  be  deeper  than 
in  anticlines,  thus  exposing  older  beds.  This  may  lead  to  confu- 


FIG.  38. — Shows  older  beds  exposed  in  synclino. 

( 

sion  as  one  naturally  expects  to  find  a  domed  condition  where  the 
oldest  beds  are  exposed;  Fig.  38  shows  this  condition.  The  beds 
at  2  are  older  than  at  1.  This  is  often  the  case  in  limestone  dis- 
tricts, as  there  is  greater  solution  action  on  the  limestone,  due  to 
the  more  plentiful  supply  of  water  in  the  synclines.  In  Cali- 


PROSPECTING  AND  MAPPING 


77 


fornia,  Wyoming  and  southeastern  Oklahoma,  in  fact,  wherever 
the  folding  is  intense,  the  occurrence  of  the  older  beds  generally 
spells  a  domed  condition,  but  such  is  not  true  by  any  means  in 
Kansas  and  Northern  Oklahoma  where  the  dips  are  very  gentle. 


FIG.  39. 


FIG.  40. 


FIG.  41. 

Fig.  39  illustrates  a  topographic  condition  that  is  found  in 
Oklahoma  at  Glenn  Pool,  the  New  York  Pool,  and  several  other 
places.  The  dip  to  the  East  may  be  called  practically  flat.  In 
such  cases  one  often  finds  a  high  escarpment  to  the  west  of  the 
field  with  low  hills  or  a  flat  bottom  to  the  east. 

Another  freak  of  erosion  is   to   find   a   " structural   high" 
eroded  on  one  side  as  shown  in  Fig.  40.     Such  occurrences  are  not 


78 


PRACTICAL  OIL  GEOLOGY 


rare  by  any  means.     A  good  example  is  the  big  dome  in  the  north 
end  of  the  Gushing  oil-field  of  Oklahoma. 


X870 


2  Miles 


FIG.  42a. — Sketch  map  showing  numbered  elevations  on  the  same  out- 
cropping stratum  at  different  points. 


2  Miles 


FIG.  426. — Same  map  as  Fig.  42a  with  structure  contour  lines  connecting 
points  of  equal  elevation  thus  outlining  an  elongated  dome. 


FIG.  42c. — Lengthwise  section  of  structure  shown  in  Fig.  426. 
(After  J.  H.  Gardner). 

In  Fig.  41,  the  bed  exposed  at  the  top  of  the  structure  is  a 
limestone.  In  this  case  the  upper  beds  that  once  covered  the  fold 
have  been  worn  away  to  the  heavy  limestone  which  was  in  time 


PROSPECTING  AND  MAPPING  79 

eroded,  leaving  a  high  surface  elevation  which  is  also  a  structural 
"high,"  i.e.j  a  dome  or  anticline.  Such  conditions  are  common 
in  Kansas  and  Oklahoma,  where  limestones  or  close-grained  sand- 
stones are  often  found  capping  the  structures. 

Vegetation. — It  is  interesting  to  note  that  in  some  localities  the 
wild  vegetation  varies  with  the  rock  below  the  soil.  Thus 
limestones  give  grassy  slopes  on  top  the  beds,  and  steep  barren 
faces;  sandstones  grow  scrub  oaks.  Again  shales  give  slopes  which 
often  carry  little  or  no  vegetation  upon  them.  Pure  shale  is 
too  compact  for  rank  vegetation. 

Structure  Contours. — Contour  lines  as  used  in  expressing 
topography  are  also  employed  in  expressing  the  geologic  struc- 
ture of  a  region  or  in  mapping  a  fold.  The  contour  lines  in 
such  cases  do  not,  however,  show  the  true  topography  of  the  sur- 
face, but  the  elevations  of  some  key  bed  of  rock  before  erosion 
took  place. 

Structure  contours  are  of  three  kinds : 

1.  Those  made  from  surface  exposures  only,  and  especially 
valuable  in  "wildcat"  territory. 

2.  Those  made  from  well  records. 

3.  Those  made  by   combining   surface   exposures    and   well 
records. 

In  all  cases  the  method  of  mapping  is  very  similar.  A  key 
bed,  characteristic  at  the  surface  or  underground,  is  chosen  as  a 
starting  point.  The  beds  above  or  below  this  key  bed  are  accu- 
rately measured  and  the  intervals  between  the  beds  noted.  The 
elevations  of  the  beds  above  sea  level  are  obtained  by  measure- 
ments with  the  aneroid  barometer,  spirit  level,  or  alidade. 

The  key  bed  at  the  surface  is  followed  along  the  outcrop,  and 
elevations  obtained  upon  it.  When  the  key  bed  is  buried,  hidden 
or  eroded  away  some  other  bed  above  or  below  the  key  bed 
is  followed.  In  the  final  checking  and  map  making,  the  beds 
are  all  referred  back  to  the  key  bed  by  adding  or  subtracting 
the  intervals  between  them.  Intervals  below  the  key  bed  must 
be  added  and  intervals  above  the  key  bed  must  be  subtracted. 

Figs.  42a,  6,  and  c  show  the  development  of  a  contour  map. 


80 


PRACTICAL  OIL  GEOLOGY 


FIG.  43a. — Shows  method  of  express-      FIG.  436. — Shows   method  of  ex- 
ing  an  anticline  by  contours.  pressing  syncline  by  contours. 


FIG.  44. — Dome  shown  by  contour  lines. 


PROSPECTING  AND  MAPPING 


81 


FIG.  45. — Anticline,  syncline,  and  monocline. 


Fault 


FIG.  46. — Contour  map  of  faulted  anticline. 
(After  U.  S.  G.  S.) 


82  PRACTICAL  OIL  GEOLOGY 

The  elevations  are  marked  on  Fig.  42a,  the  contours  are  drawn  on 
Fig.  426  and  a  cross-section,  much  exaggerated  in  vertical  scale, 
is  shown  in  Fig.  42c. 

Figs.  43a,  436,  44,  and  45  also  show  representation  of  an 
anticline  syncline,  monocline,  combination  of  the  above  three, 
and  a  dome  all  by  contour  lines. 

Such  contour  mapping  is  an  ideal  system  for  accurately  map- 
ping regions  of  low  dips  as  in  Pennsylvania  and  Oklahoma. 
There  are  places,  however,  where  the  underground  structure 
differs  materially  from  surface  structure.  Where  hidden  non- 
conformities or  faults  complicate  underground  structure,  they 
often  show  on  the  contours.  (See  Fig.  46.) 

Convergence. — If  as  in  Fig.  47  the  interval  or  distance  between 
two  beds  a  and  6  gradually  diminishes  the  beds  are  said  to  con- 
verge. This  is  due  to  unconformity,  to  overlap,  or  to  thickening 
or  thinning  of  the  beds  below  the  key  bed. 


— . 30  Miles  — 

Bed  or  Horizon 


FIG.  47. — Convergence  10  feet  per  mile  in  30  miles. 

The  best  example  in  the  Mid-Continent  field  is  the  difference 
in  interval  between  the  Oswego  limestone  and  the  Bartlesville 
sand.  In  the  Nowata  field  the  interval  is  400  ft.  At  Glenn 
Pool  50  miles  South  the  interval  is  600  ft.;  a  difference  of  200 
feet.  The  average  rate  of  thinning  to  the  north  is  4  feet  per 
mile. 

Convergence  may  cause  surface  folding  to  be  different  from 
underground  folding,  though  where  the  surface  dips  on  local 
folds  are  80  to  100  ft.  per  mile,  and  the  convergence  but  5  to 


PROSPECTING  AND  MAPPING  83 

10  ft.  per  mile,  the  underground  structure  will  be  only  slightly 
influenced  by  convergence;  and  for  all  practical  purposes  con- 
vergence can  be  neglected. 

The  construction  of  a  convergence  sheet  and  its  application  is 
explained  below.  The  text  is  taken  from  Bulletin  318,  United 
States  Geological  Survey,  by  W.  T.  Griswold  and  M.  J.  Munn. 

Construction  of  Maps. — The  work  of  making  a  map  of  a  par- 
ticular stratum  lying  at  a  considerable  depth  below  the  surface 
consists  of  three  distinct  steps — first,  careful  contour  mapping  of 
some  prominent  surface  beds,  called  the  "key  horizon;"  second, 
the  more  difficult  task  of  ascertaining  the  distance  between  this 
key  horizon  and  the  producing  oil  sand  below  and  the  amount  and 
direction  of  the  variation  in  this  distance;  third,  the  application  of 
a  correction  to  the  surface  mapping  equal  to  this  convergence,  so 
that  lines  drawn  on  this  map  connecting  points  of  equal  elevation 
above  the  sea  (contour  lines)  will  show  the  true  shape  of  the  sur- 
face of  the  oil  sand. 

Structural  Map  of  the  Key  Horizon. — On  the  completion  of  the 
field  work,  as  previously  described,  the  geologist  had  a  topo- 
graphic map  of  the  area  on  which  the  horizontal  location  and  the 
elevation  of  the  outcrops  of  different  marking  strata  are  shown 
at  hundreds  of  points.  (See  Fig.  48.)  By  a  comparison  of  these 
outcrops,  the  intervals  between  different  marking  beds  were 
obtained.  One  bed  was  selected  ^as  the  key  horizon,  usually  that 
outcropping  over  the  greatest  area.  By  adding  to  or  subtracting 
from  the  elevation  of  outcrops  of  other  known  beds  the  distance 
they  have  been  found  to  be  below  or  above  the  key  horizon,  the 
elevation  of  that  stratum  was  obtained  at  a  great  many  points. 
By  drawing  lines  connecting  the  points  of  equal  elevation,  a  con^ 
tour  map  of  the  key  horizon  was  produced. 

Convergence  Map. — A  knowledge  of  the  variation  in  distance 
between  the  key  horizon  and  the  oil  sand  was  gained  from  the 
records  of  wells  in  different  parts  of  the  area,  and  without  these 
records  it  would  be  impossible  to  make  any  illustration  that  would 
show  the  form  and  position  of  the  sand,  unless  it  were  exactly 
parallel  with  the  key  horizon. 


84 


PRACTICAL  OIL  GEOLOGY 


FIG.  48.— Contour  Map  of  Key  Horizon,  which  in  this  case  is  the 
Pittsburgh  Coal. 


AND  MAPPING 

1I»W  notnomb3 


Edmonton  Well 


FIG.  49. — Convergence  sheet  of  Fig.  48  shows  distance  from  Pittsburgh 
Coal  to  top  of  Berea  Oil  Sand. 


PROSPECTING  AND  MAPPING 

R.  2  W.  40' 


85 


>il  Horizon,  which  in  this  case  i 
il  Sand. 


86  PRACTICAL  OIL  GEOLOGY 

To  make  use  of  the  well  records  and  construct  an  actual  map 
of  the  oil-bearing  sand,  the  following  method  was  employed:  On 
the  map  of  the  structure  of  the  key  horizon  were  plotted  all  the 
wells  drilled  within  the  area.  As  the  elevation  of  the  mouth  of 
each  of  these  wells  had  been  determined,  the  position  of  the  key 
stratum  with  reference  to  the  mouth  of  the  well  was  obtained 
directly  from  the  map,  and  with  this  information  the  distance 
from  the  key  stratum  to  the  oil-bearing  sand  at  this  point  was 
obtained  from  the  record  of  the  well. 

By  making  this  computation  for  each  well  of  which  a  reliable 
record  could  be  obtained,  the  distance  from  the  key  horizon  to 
the  oil-bearing  stratum  was  obtained  in  different  parts  of  the 
area.  Generally  this  distance  is  not  the  same  at  different  wells, 
but  decreases  in  one  direction  or  the  other. 

The  correction  for  the  convergence  between  the  key  horizon 
and  the  oil  sand  is  applied  by  means  of  a  mechanical  drawing 
called  a  "convergence  sheet."  (See  Fig.  49.)  This  drawing  was 
made  on  tracing  cloth  by  connecting  the  location  of  the  oil  wells 
from  which  reliable  records  had  been  obtained  by  straight  lines. 
The  lines  were  then  ""divided  proportionately  to  the  amount  of 
convergence  found  between  the  two  wells,  so  that  each  division 
on  the  lines  would  represent  the  increased  distance  of  10  ft. 
between  the  key  stratum  and  the  oil-bearing  sand. 

After  all  the  lines  connecting  the  different  wells  had  been  thus 
divided,  the  points  that  show  an  equal  distance  from  the  key 
stratum  to  the  oil  sand  were  connected,  and  a  drawing  was  built 
up  that,  when  placed  over  the  map  on  which  the  elevation  of  the 
key  stratum  was  noted  at  many  different  places,  showed  directly 
what  distance  should  be  subtracted  from  each  elevation  of  the 
key  stratum  to  make  it  equivalent  to  the  elevation  of  the  oil 
sand  at  that  point. 

The  regularity  and  uniformity  of  this  mechanical  drawing 
shows  whether  it  is  possible,  or  not,  to  make  a  map  of  the  oil  sand 
that  will  be  of  any  practical  .value.  If  the  distance  between 
the  10-ft.  lines,  which  are  called  isochor  (equal  space)  lines,  is 
regular  and  the  decrease  is  uniformly  in  one  direction,  a  map  of  the 


PROSPECTING  AND  MAPPING  87 

lower  sands  can  be  made  practically  correct.  If,  however,  the 
distance  from  the  key  horizon  to  the  sand  decreases  first  in  one 
direction  and  then  in  another,  the  lines  on  the  convergence  sheet 
will  run  in  circles  and  show  that  there  is  little  use  in  trying  to 
interpret  the  structure  of  the  sand  from  a  map  of  the  surface 
structure.  It  can  hardly  be  hoped  that  the  wells  used  have  been 
located  at  the  exact  point  of  the  greatest  distance  between  the 
two  strata.  In  all  probability  the  resulting  convergence  is  in- 
correct over  limited  areas. 

The  amount  of  convergence  per  mile  is  another  condition  to 
be  considered.  If  it  amounts  to  50  or  60  ft.  to  the  mile,  there 
is  little  probability  that  the  resulting  map  of  the  sand  will  be 
correct  within  a  limit  of  20  to  30  ft.  If,  however,  the  conver- 
gence is  only  10  or  20  ft.  to  the  mile,  the  resulting  map  should 
be  of  the  same  degree  of  accuracy  as  the  map  of  the  surface 
structure. 

In  making  maps  of  subsurface  strata  in  areas  that  have  not 
been  productive,  most  of  the  records  used  for  making  a  conver- 
gence sheet  must  be  taken  from  " wild-cat"  wells.  In  certain 
cases  it  is  difficult  to  procure  the  records  of  such  wells,  and  often 
the  best  that  can  be  obtained  is  the  depth,  from  memory,  at 
which  the  sand  was  found.  Here  is  a  source  of  serious  error,  for 
a  mistake  in  this  distance  may  make  the  resulting  map  incorrect 
for  a  considerable  distance  about  the  well. 

In  making  a  subsurface  map,  full  knowledge  should  be  had  of 
the  well  records  used  for  constructing  the  convergence  sheet,  and 
if  any  reliable  records  have  been  thrown  out  whose  distances 
would  change  the  convergence  sheet,  the  reason  for  discarding 
them  should  be  given.  In  selecting  the  records  for  the  construc- 
tion of  a  convergence  sheet  it  is  desirable  to  consider  wells  from 
which  a  good  record  is  obtainable,  and  those  that  are  located  near 
the  outcrop  of  an  easily  recognized  surface  stratum. 
.  Map  of  Oil  Sand. — With  the  convergence  sheet  completed,  the 
operation  of  making  a  contour  map  of  the  oil  sand  is  very 
simple.  The  tracing  is  placed  over  the  map  on  which  are  noted 
the  elevations  of  the  key  horizon.  From  each  of  these  elevations 


88 


PRACTICAL  OIL  GEOLOGY 


the  amount  shown  by  the  convergence  sheet  was  subtracted. 
This  gave  the  elevation  of  a  point  on  the  oil  sand.  By  connecting 
the  points  of  equal  elevation  by  lines,  a  contour  map  of  the 
oil-bearing  sand  was  made.  (See  Fig.  50.) 

Geological  Map. — The  accompanying  geological  map  and  cross- 
sections  (see  Figs.  51  and  52)  illustrate  the  structure  of  a  typical 
region.  Symbols,  a  fault,  and  several  formations  are  shown.  A 
thorough  study  of  this  map  and  the  cross-sections  will  enable  one 
to  comprehend  the  U.  S.  Geological  Survey's  geological  maps. 


CJL4i^^;%««*y;;;j;;;y5«&"s';!!ix«5i?x^«;.t. 
±*°-    -^^l^^^^^l^ 


FIG.  51. — Geologic  map. 

Contact  lines  are  lines  marking  the  boundary  of  geological 
formations  of  varying  ages,  and  on  maps  are  represented  as  in 
Fig.  51.  Other  symbols  are  used  to  suit  varying  conditions. 

Colored  maps  and  cross-hatching  are  generally  used  to  make 
mapping  clear.  The  table  (see  Fig.  53)  explains  the  symbols 
used  and  followed  in  this  book. 

Photography. — The  use  of  photographs  to  illustrate  structures 
is  very  useful  where  the  folds  are  small  enough  to  be  readily 


PROSPECTING  AND  MAPPING 


89 


FIG.  52.— Cross-sections  along  lines  A-A',  B-B',  C-C'  in  Fig.  51. 


MAP  SYMBOLS 

y^O*        Strike  and  Dip 

1  Anticline 
<    I  >       Dome 

-t                 C!-irnf>1iTV/i 

Gravel,  Conglomerate 
Sand 
Sandstone 
Clay 
Shale 
Limestone 

Igneous  Rocks 

Gas  Well 
Abandoned  Well 
Dry  Hole 

SSI 



rj^.i_nr: 

—Fault 
Contact 

?   Overturn 

oof 

X          Seepage 
•          Oil  Well. 
©          Drilling  Well 

• 

IL-~~ 

1      1 

1      1 

1     1 

* 

V 

X 

FIG.  53. — Symbols  used  in  mapping. 


90 


PRACTICAL  OIL  GEOLOGY 


O2 

d 

OQ 


ft 

| 

bfl 

| 

ft 

•g 


PROSPECTING  AND  MAPPING 


91 


photographed.  An  illustration  of  such  usefulness  is  given  in 
Fig.  54  in  which  a  photograph  and  a  sketch  from  the  photograph 
both  depict  structural  conditions. 

Stereograms  and  Cross-sections. — Stereograms  are  graphic 
models  of  structure  and  are  sometimes  used  to  explain  under- 
ground conditions.  (See  Fig.  55.)  The  figures  in  Chapter 
IV  show  a  means  of  explaining  underground  structure  by  using 
cross-sections. 

Models. — Miniature  models  like  those  used  by  architects  in 
depicting  large  buildings  are  coming  into  use  to  illustrate  under- 
ground conditions.  Models  have  also  been  employed  in  mining 
work  for  some  years  but  their  use  in  oil  fields  has  not  become 
general. 


FIG.  55. — Stereogram  of  plunging 
anticline. 


FIG. 


56. — Strike    and 
Geikie.) 


dip.      (After 


The  employment  of  models  is  simply  a  graphic  method  which 
simplifies  explanations  to  the  layman,  and  enables  the  practical 
operator  to  see  conditions  at  a  glance.  In  using  this  method, 
miniature  representations  of  the  surface  and  of  the  oil  and  of  the 
water  sands  are  constructed  of  paper,  of  wood,  or  of  wire  and 
sometimes  enclosed  in  glass  cases.  Such  models  take  considerable 
time  to  make,  and  require  considerable  cost  but  are  excellent  for 
class-room  studies,  for  illustrating  legal  phases  of  oil-field  practice, 
and  for  use  in  explaining  properties  to  stockholders,  directors, 
prospective  investors,  etc. 

Dip  (See  Fig.  56). — The  inclinations  given  to  the  beds 
forming  the  earth's  surface  are  called  dips  and  are  measured 


92  PRACTICAL  OIL  GEOLOGY 

by  a  clinometer.  Dips  are  expressed  in  degrees  measured  from 
the  plane  of  the  horizon,  such  as  20°,  45°,  90°.  Sometimes  they 
are  expressed  as  slopes  of  17  per  cent.,  25  per  cent.,  etc.,  which  is 
the  same  system  as  used  in  road  and  street  construction  (see 
Table  XI)  and  is  very  convenient  in  structural  contouring.  • 

In  Pennsylvania,  Ohio,  and  Oklahoma,  dips  not  exceeding 
30  ft.  per  mile  are  common.  A  slope  of  40  ft.  per  mile  is 
sufficient  to  localize  the  petroleum  in  some  fields.  In  the 
Pennsylvania  fields  250  ft.  per  mile  is  considered  a  steep  dip  or 
slope.  The  gentler  dips  of  the  Eastern  and  Mid-continent 
fields  are  in  direct  contrast  to  the  steep  dips  of  California. 
Dips  of  45°  to  90°  are  not  at  all  uncommon  in  many  California 
fields.  Dip  per  100  ft.  is  often  used  instead  of  dip  per  mile,  as 
the  changes  in  slope  are  generally  very  rapid  and  will  not  gen- 
erally continue  uniform  for  distances  over  J^  mile,  and  in  many 
cases  not  so  far. 

In  reading  dips  the  tendency  is  to  overestimate,  rather  than 
to  underestimate  the  amount  of  dip'.  This  is  true  because  the 
surface  beds  often  weather  and  slide,  showing  exposures  with 
greater  dips  than  the  true  dips.  The  most  accurate  dips  are  those 
obtained  by  sighting  over  considerable  distance,  say  one-fourth  to 
several  miles.  The  dips  of  minor  folds  are  sometimes  observed 
and  mistaken  for  the  dip  of  major  structures.  One  must 
especially  guard  against  confusing  such  dips. 

Strike  (See  Fig.  56). — "A  horizontal  line  drawn  at  right  an- 
gles to  the  dip  is  called  the  strike  of  the  rocks.  Strike  may  be 
conceived  as  always  a  level  line  on  the  plane  of  the  horizon,  so 
that  no  matter  how  much  the  group  may  undulate,  or  the  out- 
crop may  vary  or  the  dip  may  change,  the  strike  will  remain 
level  or  horizontal."  The  direction  of  strike  is  expressed  in 
directions  of  the  compass  as  N.  25  E.  or  S.  85  W. 

Earth  Curves. — Remember  that  all  folds  are  parts  of  earth 
curves.  For  this  reason  a  dip  at  one  point  on  the  curve  in- 
creases or  decreases  at  other  points.  As  the  locus  or  central 
point  of  this  curve  is  somewhere  in  the  center  of  the  fold,  the 
dips  along  a  horizontal  line  will  show  different  values  for  each 


PROSPECTING  AND  MAPPING 


93 


stratum.     A  knowledge  of  this  fact  shows  that  dips  must  be  used 
very  carefully  to  obtain  accurate  results. 

Plunge. — Plunge  is  the  pitch  of  the  whole  formation  along 
the  strike.  Thus  in  anticlinal  domes  there  is  a  pitch  along 
the  strike  of  the  anticline.  This  pitch  is  called  the  plunge  to 
distinguish  it  from  the  dip,  at  right  angles  to  the  strike.  Both 
dip  and  plunge  are  measured  in  a  similar  manner.  (Fig.  55 
illustrates  a  plunging  anticline.) 

TABLE  X. — CONVERSION  OP  PER  CENT.  GRADE  TO  ANGULAR 
INCLINATION 

(After  Hayes) 


Per  cent, 
grade 

Angular 
inclination 

Per  cent, 
grade 

Angular 
inclination 

Per  cent, 
grade 

Angular 
inclination 

1.0 

35' 

7.00 

4° 

13.00 

7°  25' 

1.50 

52' 

7.50 

4°  15' 

14.00 

8° 

1.75 

1° 

8.00 

4°  35' 

15.00 

8°  30' 

2.00 

1°  10' 

8.50 

4°  50' 

15.85 

9° 

2.50 

1°  25' 

8.75 

5° 

16.00 

9°     5' 

3.00 

.1°  45' 

9.00 

5°  10' 

17.00 

9°  40' 

3.50 

"  2° 

9.50 

5°  25' 

17.65 

10° 

4.00 

2°  15' 

10.00 

5°  50' 

18.00 

10°  15' 

4.50 

2°  35' 

10.50 

6° 

19.00 

10°  45' 

5.00 

2°  50' 

11.00 

6°  15' 

19.45 

11° 

5.25 

3° 

11.50 

6°  35' 

20.00 

11°  20' 

5.50 

3°  10' 

12.00 

6°  50' 

21.00 

11°  50' 

6.00 

3°  25' 

12.25 

7° 

21.25 

12° 

6.50 

3°  45' 

12.50 

7°  10' 

CHAPTER  VI 

LOCATING  DRILL-HOLE  SITES 
LOCATING  SITES  FOR  TEST  HOLES 

The  location  of  drill-hole  sites  depends  upon  many  factors 
which  for  convenience  may  be  divided  into  two  classes : 

(1)  Those  where  structural  features  are  evident;  as  where  out- 
croppings  are  in  evidence,  and  where  one  finds  oil  seepages,  as- 
phaltum  residues,  etc. 

(2)  Those  where  structural  features  are  unknown;  as  where 
structural  features  are  covered  up,  though  gaseous  emanations, 
oil  on  spring  or  lake  waters,  and  other  oil  signs  may  be  known. 

Where  structural  features  are  favorable  for  oil,  one  generally 
finds  some  oil  signs.  Where  dips  are  known  and  structural 
features  are  outlined,  locating  becomes  merely  a  question  of 
judgment,  governed  by  the  following  mechanical  and  geological 
factors : 

(1)  Degree  of  dip  of  strata.  (2)  Thickness  of  overlying  cover. 
(3)  Deformation  of  strata,  i.e.,  sudden  change  in  dips  due  to: 
(a)  faulting,  and  (6)  minor  folds. 

Locations  Where  Outcroppings  are  Evident. — Rule. — Locate 
a  test  well  down  the  dip  (see  Fig.  57)  from  an  oil  seepage,  or 
other  oil  sign  at  the  outcropping  of  oil  sand.  This  simple  rule 
has  often  been  disobeyed  with  a  consequent  loss  of  thousands  of 
dollars. 

In  many  cases,  drill  holes  are  started  above  the  outcrop  of  the 
oil  sand.  Negative  results  were  thus  assured  from  the  beginning 
in  the  top  sand,  though  production  was  sometimes  obtained  in 
the  sand  below. 

Where  outcrops  are  not  in  evidence,  or  the  desire  is  to  drill  for 
oil  measures  other  than  those  outcropping,  the  rule  given  above 

94 


LOCATING  DRILL-HOLE  SITES 


95 


cannot  be  applied.  In  such  cases,  the  structural  features  (anti- 
clines, or  some  modifications  of  anticlines)  are  carefully  deter- 
mined by  surveys. 

When  the  surface  axis  of  the  anticline  has  been  determined, 


FIG.  57. — Shows  effect  of  topography  in  choosing  well  sites. 

the  dips  of  the  sides  or  flanks  must  be  known  to  determine  the 
shape  of  the  anticline  and  the  inclination  or  dip  of  the  beds  away 
from  the  axis. 

If  the  anticline  is  symmetrical,  place  the  test  hole  on  the  axis. 


w 


FIG.  58. — Illustrates  locating  wells  on  an  asymmetrical  anticline. 

If  one  flank  of  the  anticline  is  much  steeper  than  the  other, 
place  the  wells  on  the  flank  of  less  inclination.  (See  Fig.  58.)  It  is 
readily  seen  in  this  case  that  the  east  slope  gives  a  wide  expanse 
of  comparatively  shallow  territory,  and  that  the  west  slope 


96  PRACTICAL  OIL  GEOLOGY 

gives  a  narrow  belt  that  rapidly  becomes  very  deep.  The  under- 
ground axis  of  each  oil  sand  lies  further  east  than  the  axis  of 
the  sand  above.  By  drilling  on  the  gentler  slope  there  is  less 
liability  of  missing  any  oil  sands  than  by  drilling  on  the  west 
slope.  Bear  in  mind  in  all  the  above  cases  that  the  apex  or  high 
point  of  the  simple  anticline,  or  the  apices  of  domes,  are  to  be 
chosen  in  preference  to  the  low  places  on  the  axis  of  the  fold.  In 
other  words  the  top  of  the  fold  across  the  line  of  strike  is  not  alone 
a  sufficiently  good  place  on  which  to  locate  a  well  but  the  position 
along  the  strike  must  also  be  taken  into  consideration. 

It  is  not  always  best  to  locate  the  first  well  directly  on  the 
apex  of  the  structure  for  in  some  cases  such  domes  are  unsatu- 
rated  with  oil  at  the  apices  though  gas  occurs  there.  To  obtain 
oil  locate  further  down  the  dip  or  the  plunge  of  the  structure. 

Depths  of  Prospect  Holes. — The  distance  of  a  drill  hole  from 
the  outcrop  should  be  such  that  oil  may  be  reached  at  a  minimum 
depth  and  at  least  cost. 

One  should  not  expect  sensational  results  from  a  prospect  hole. 
Ten  to  twenty  barrels  of  light  gravity  oil  (25°  and  over)  in  shallow 
territory,  500  to  1500  ft.  in  depth,  proves  that  the  field  has  com- 
mercial value,  if  one  can  gain  this  depth  at  a  moderate  cost.  The 
depth  of  the  hole  varies  with  the  dip  and  the  distance  from  the 
outcrop. 

Where  there  is  an  added  overburden  of  a  thousand  feet  or 
more,  as  is  the  case  in  some  fields,  depths  become  much  greater 
and  drilling  more  expensive.  This  question,  however,  will  be 
more  fully  discussed  below. 

Effect  of  Topography  on  Depth  to  Oil. — The  differences  in 
thickness  of  beds  overlying  the  oil  formation  has  a  distinct  effect 
on  depths  to  the  oil  sand,  as  shown  in  Fig.  57.  A  indicates  the 
outcropping  of  the  sand,  the  angle  25°  shows  the  dip  of  the 
strata,  and  the  line  A-B  corresponds  to  the  horizon.  Without  the 
additional  overburden  above  the  line  A-B  the  depths  would  be 
influenced  solely  by  the  dip,  and  the  distances  from  the  out- 
cropping. The  drill  hole  at  4  reaches  the  oil  sands  at  less 
depth  than  at  3  and  5.  Simple  as  these  cases  may  seem,  it  is 


LOCATING  DRILL-HOLE  SITES 


97 


by  no  means  unusual  to  see  wells  located  on  hill  tops  when  they 
would  have  the  advantage  of  accessibility  and  would  be  shallower 
when  located  in  canons  or  ravines.  It  is,  of  course,  imprudent  to 
select  a  site  in  the  bottom  of  a  gully  where  the  torrents  would  be 
liable  to  wash  away  the  rig,  but  place  the  rig  on  one  side  of  the 
ravine,  where  it  would  be  out  of  a  torrent's  path. 

Erosion. — Erosion  determines  to  a  great  degree  the  location  of 
a  test  well.  By  erosion  is  meant  the  wearing  down  of  the  earth's 
surface  due  to  the  action  of  rivers,  rains,  winds,  frost,  and 
chemicals  in  the  air.  In  some  oil  fields,  deep  canons  have  been 
cut  through  the  oil  formations  and  have  resulted  in  draining  the 


FIG.  59. — Shows  how  erosion  exposes  oil  sands  to  drainage. 

oil  from  the  oil  strata.  The  San  Juan  oil  field  of  Utah  is  supposed 
to  have  been  drained  by  the  San  Juan  river  that  cuts  a  canon 
through  the  anticline,  exposing  several  oil  strata.  (See  Fig.  59.) 
In  locating  wells  in  such  a  field  drill  down  the  dip  from  the  ex- 
posed strata.1 

Deformations. — FAULTS. — Under  this  heading  we  will  consider 
sudden  changes  in  dip  due  to  faulting,  and  to  minor  folds.  Per- 
haps no  question  dealing  with  locating  involves  more  com- 
plexities than  the  problems  of  faulting.  Faults  are  so  irregular 
and  numerous  in  some  fields  that  they  become  important  factors 
in  choosing  well  sites.  Fig.  29a,  Chapter  IV,  shows  a  thrust  fault 

1  Light  gravity  oils  with  a  paraffine  base  may  escape  and  leave  the  strata 
barren. 


98 


PRACTICAL  OIL  GEOLOGY 


involved  with  folding.  Such  a  sketch  is  valuable  to  the  man  who 
wants  to  know  why  oil  is  not  found  in  certain  localities.  Wells  at 
1  would  and  did  prove  barren,  while  those  toward  the  right  were 
productive. 


FIG.  60. — Illustrates  how  the  upthrow  side  of  a  fault  acts  as  an  anticline; 
the  downthrow  side,  as  a  syncline. 

Again,  in  Fig.  60,  there  is  a  normal  fault,  showing  the 
influence  of  such  a  fault  on  the  oil  strata.  In  this  case  the  up- 
throw side  of  the  fault  on  the  west  acts  as  an  anticline,  the  down- 


FIG.  61. — Illustrates  how  normal  faulting  may  leave  barren  spaces. 

throw  side,  as  a  syncline.     Wells  1  and  3  obtained  oil;  2  and  4 
did  not. 

In  Fig.  61  a  fault  is  shown  that  illustrates  how  a  drill  hole 
might  pass  between  productive  oil  strata  and  fail  to  strike  oil 


LOCATING  DRILL-HOLE  SITES 


99 


in  commercial  quantities.  The  shale  on  the  hanging-wall  side 
abuts  against  the  oil  sands  on  the  footwall  side,  and  confines 
the  oil  to  the  sands  below. 

Faults  do  not  necessarily  affect  oil  formations  adversely,  as 
shown  in  Figs.  27  and  296,  Chapter  IV.  Careful  study  will  show 
that  in  many  cases  faults  are  beneficial  rather  than  detrimental. 

Min0r  Folds. — Minor  folds  have  a  very  decided  effect  in  some 
regions.  Fig.  62  shows  a  number  of  minor  folds  on  the  flank  of 
a  monocline.  These  folds  must  be  taken  into  consideration  in 
drilling.  Thus  wells  placed  at  various  points,  as  at  1,  3,  and  5, 
would  strike  oil  and  gas  at  much  less  depths  than  the  major 
slope  indicates.  In  such  cases,  too,  one  might  strike  water 
sands  in  the  lower  arches  as  at  2,  4,  and  6  and  consider  the  field 


FIG.  62. — Shows  lower  fold  non-productive;  folds  higher  on  slope  increase 

in  quantity  of  oil. 

as  barren.  Such  a  conclusion  would  be  wrong.  A  knowledge 
of  such  minor  folds  is  of  great  importance  in  locating  wells  in 
all  oil  fields. 

Twin  Anticlines. — The  two  anticlines  shown  in  Fig.  63  are 
really  minor  crumples  on  one  big  fold.  However,  one  is  apt 
to  be  greatly  misled  by  the  minor  crumples.  By  placing  a  well 
at  1,  the  lower  oil  sand  was  missed  entirely,  while  in  a  well  at 
2,  the  lower  oil  sand  was  reached  at  a  moderate  depth.  The 
true  apex  of  such  a  fold  is  at  A,  and  the  axis  is  inclined  as  shown. 
Broad  flat-topped  folds  of  this  type  are  very  deceptive. 

Disappearing  Folds. — In  many  cases,  especially  where  soft 
. shales,  clays,  sands,  and  gravel  overlie  harder  beds,  surface 


100 


PRACTICAL  OIL  GEOLOGY 


folds  disappear  at  shallow  depths.  The  wrinkles  on  the  surface 
die  out  as  shown  in  Fig.  64.  Where  this  is  the  case,  no  localiza- 
tion of  the  oil  would  take  place  at  depth.  Drilling  for  petroleum 
would  not  be  advised  under  such  conditions. 

Joint  Planes. — At  Florence,  Colorado,  petroleum  occurs  along 
joint  or  small  fault  planes  on  a  low  westward  dipping  monocline. 
Wells  drilled  into  the  joint  planes  obtain  oil,  but  if  the  wells 
miss  a  crevice,  oil  is  not  obtained  in  commercial  quantities.  If 
two  wells  strike  the  same  joint  plane,  one  well  will  generally  affect 
the  production  of  the  other  by  lowering  the  gas  pressure,  or  by 
decreasing  the  oil  production.  Locating  in  such  cases  is  very 
difficult. 


Well 
Location 


FIG.  63. — Illustrates  well  loca- 
tions on  minor  folds  of  a  highly 
compressed  structure. 


FIG.  64. — Illustrates  a  minor  fold 
disappearing  with  depth. 


Locating  Wells  on  Domes. — On  anticlinal  domes,  select  the 
high  points  or  crests  of  the  domes  on  which  to  place  the  wells. 
Where  such  domes  are  unsaturated  with  water,  oil  will  not  occur 
on  the  apex  but  around  it.  Such  unsaturated  domes  occur  in 
some  of  the  Pennsylvania  and  Wyoming  fields. 

With  volcanic  necks,  the  wells  must  be  started  to  miss  the 
volcanic  rocks.  (See  Fig.  23,  Chapter  IV.) 

With  saline  domes,  the  oil  is  found  overlying  the  salt  core 
and  to  the  side  of  it  in  many  cases.  However,  careful  work  is 
necessary  to  determine  well  locations  in  such  fields,  and  often 
the  only  way  is  to  drill  and  find  out  the  underground  condi- 
tions. It  has  been  found  that  drilling  in  the  center  of  these 


LOCATING  DRILL-HC&E  SITES  10  ]/ 

domes  generally  lands  the  hole  in  a  salt  core  instead  of  an  oil- 
producing  formation.  The  best  practice  is  to  locate  the  test 
holes  on  the  slopes  of  the  domes,  several  hundred  feet  from 
the  center,  thus  missing  the  salt  core.  (See  Fig.  24,  Chapter  IV.) 
Wells  under  Ocean  or  Lake. — It  has  been  often  asked  why 
oil  should  be  found  under  the  ocean.  At  Summerland,  Santa 
Barbara  county,  California,  a  good  illustration  is  found.  Fig. 
65  roughly  illustrates  the  existing  conditions.  Wells  are  there 
located  on  piers  extending  into  the  ocean.  In  drilling,  the 
ocean  water  is  cased  off,  and  drilling  proceeds  much  as  on  dry 
land. 


Shore 


FIG.  65. — Illustrates  oil  out  in  ocean;  derricks  on  piers. 

Effects  of  Steep  Dips  (See  Fig.  66). — Steep  dips  increase 
depths  rapidly.  Wells  placed  at  1  will  strike  the  sand  close  to 
the  surface,  but  at  3  and  4  will  deepen  rapidly.  The  productive 
area  will  here  necessarily  be  narrow  and  confined  to  the  crest  of 
the  anticline. 

Types  of  such  anticlines  are  found  in  the  Olinda  oil  fields  of 
California,  the  Los  Angeles  field,  the  Pico  anticline  of  Los 
Angeles  county,  and  the  Modelo  anticline  of  Ventura  county, 
California. 

In  such  anticlines,  the  apparent  thickness  of  the  oil  sands  is 
very  great  as  the  drill  travels  through  the  beds  for  a  much 
greater  distance  than  if  the  beds  lay  more  nearly  horizontal. 


1O2 


PRACTICAL  OIL  GEOLOGY 


By  drilling  a  short  distance  off  the  axis,  one  may  easily  miss 
the  productive  strata.  (See  4,  Fig.  66.) 

Where  steep-dipping  oil-bearing  formations  occur  uncovered 
(see  Fig.  66),  there  is  a  better  chance  of  the  oil  being  retained 
than  in  strata  where  the  dips  are  gentle.  This  is  especially 
true  in  arid  regions  where  the  rainfall  is  small.  The  steeper 
the  dip  the  less  the  chance  of  catching  water,  as  the  runoff  is 
large  and  the  surface  exposed  small. 

Such  highly  compressed  folds  are  often  fractured  and  faulted 
along  the  line  of  the  axis  so  that  seepages  are  common;  and 
oil  may  also  find  more  porous  reservoirs  there. 


FIG.  66. — Illustrates  locating  of  wells  on  steep  dips. 

Locating  Wells  on  Terraces. — On  terrace  structure,  the  flat 
part  of  the  structure  carries  the  oil.  To  obtain  the  best  results 
locate  a  well  on  the  flat  part  of  the  structure  away  from  the  sur- 
face axis,  as  at  1  and  2.  (See  Fig.  20a.)  (East  of  axis  in  this 
case.) 

With  a  terrace  the  axis  dips  with  depth,  so  the  axis  of  the  oil 
stratum  will  not  lie  immediately  below  the  surface  axis  but  at  an 
angle  to  it.  (See  Fig.  20a.) 

Locating  Where  Structural  Features  are  not  Evident. — We  have 
so  far  considered  locating  where  evidence  of  structural  features 
are  known.  Where  out-croppings  are  not  in  evidence  drilling 
is  necessarily  an  uncertain  process.  In  such  cases,  one  must  be 
guided  by  local  experience  in  drilling  for  water,  by  abandoned  oil 
tests,  by  mine  records  (if  any  are  available)  and  any  evidence  of 
"oil  sign"  that  can  be  found.  Necessarily  one  must  be  prepared 


LOCATING  DRILL-HOLE  SITES 


103 


to  drill  to  a  considerable  depth  in  such  cases.  No  estimates  of 
probable  depth  to  reach  oil  are  trustworthy  here,  and  under  such 
conditions  a  well  is  a  gamble. 

When  capital  is  available,  several  drill  holes  may  be  put  down ; 
if  the  first  one  proves  barren,  the  second  may  succeed.  Indeed 
it  is  not  unusual  to  find  oil  in  regions  that  have  been  declared 
barren  by  earlier  prospectors. 

So  far,  we  have  considered  the  factors  involved  in  the  location 
of  oil-well  sites.  The  next  chapter  will  consider  some  of  the 
factors  in  actual  drilling  operations. 


CALCULATIONS  FOR  DEPTHS   TO    OIL   SANDS 

Method  1. — In  Fig.  67  the  elevation  of  the  seepage  above  sea 
level  is  given  as  1000  ft.  and  the  elevation  of  the  well  site  as  1425. 
The  difference  in  elevations  is  1425  -  1000  =  425,  or  bd  =  E. 


FIG.  67. — Method  of  estimating  the  dip  of  a  bed. 

The  horizontal  distance  between  the  two  points  is  ab,  or  here 
1320  ft. 

The  angle  of  dip,  0,  of  the  oil  formation  is  25°. 

The  height  be  is  desired. 

To  find  the  total  depth  H,  add  be  to  E. 


104 


PRACTICAL  OIL  GEOLOGY 


BY  TRIGONOMETRY: 

Formula:   Tan  0  X  (a&)  +  E  =  H. 
Tan  25°  X  1320  =  be. 

Tan  25°  (obtained  from  a  log  table)  =  .4663. 
.4663  X  1320  =  616  (approx.)  =  be. 
H  =  616  +  425  =  1041  ft.,  the  depth  to  the  sand. 
Such  depths  are  approximate  as  one  does  not  always  know  of 
changes  in  thickness  of  formations  or  in  dips  underground. 

TABLE  XI. — DEPTH  TO  A  STRATUM  BELOW  THE  HORIZONTAL    SURFACE 

FOR  VARIOUS  DISTANCES  AND  DIPS 

(After  Hayes) 


Angle 
of  dip 

Feet 

I/4 
mile 
(1320 
ft.) 

1/2 
mile 
(2640 
ft.) 

3/4 
mile 
(3960 
ft.) 

mile 
(5280 
ft.) 

100 

200 

300 

400 

500 

1000 

1 

1.75 

3.50 

5.25 

7.00 

8.75 

17.5 

23.04 

46.08 

69.12 

92.16 

2 

3.49 

6.98 

10.47 

13.96 

17'.45 

34.9 

46.09 

92.18 

138.3 

184.4 

3 

5.24 

10.48 

15.72 

20.96 

26.20 

52.4 

69.18 

138.4 

207.5 

276.7 

4 

6.99 

13.98 

20.97 

27.96 

34.95 

69.9 

92.30 

184.6 

276.9 

369.2 

5 

8.75 

17.50 

26.25 

35.00 

43.75 

87.5 

115.5 

230.5 

346.5 

461  .9 

6 

10.51 

21.02 

31.53 

42.04 

52.55 

105.1 

138.7 

277.4 

416.1 

555.0 

7 

12.28 

24.56 

36.84 

49.12 

61  .40 

122.8 

162.1 

324.2 

486.3 

648.3 

8 

14.05. 

28.10 

42.15 

56.20 

70.20 

140.5 

185.5 

371  .0 

556.5 

742.0 

9 

15.84 

31.68 

47.52 

63  .36 

79.20 

158.4 

209.1 

418.2 

627.3 

836.3 

10 

17.63 

35.26 

52.89 

70.52 

88.15 

176.3 

232.8 

465.6 

698.4 

931.0 

11 

19.44 

38.88 

58.32 

77.76 

97.20 

194.4 

256.6 

513.2 

769.8 

1026.0 

12 

21  .26 

42.52 

63.78 

85.04 

106.30 

212.6 

280.6 

561.2 

841.8 

1123.0 

13 

23.09 

46.18 

69.27 

92.36 

115.45 

230.9 

304.7 

609.4 

914.1 

1219.0 

14 

24.93 

49.86 

74.79 

99.72 

124.65 

249.3 

329.1 

658.2 

987.3 

1316.0 

15 

26.80 

53.60 

80.40 

107.20 

134.00 

268.0 

353.7 

707.4 

1060.0 

1415.0 

16 

28.68 

57.36 

86.04 

114.72 

143.40 

286.8 

378.5 

757.0 

1136.0 

1514.0 

17 

30.57 

61  .14 

91.71 

122.28 

152.85 

305.7 

403.6 

807.2 

1211  .0 

1614.0 

18 

32.49 

64.98 

97.47 

129.96 

162.45 

324.9 

428.9 

857.8 

1287.0 

1716.0 

19 

34.43 

68.86 

103.29 

137.72 

172.15 

344.3 

454.3 

908.6 

1363.0 

1817.0 

20 

36.40 

72.80 

109  .20 

145  .60 

182  .00 

364.0 

480.4 

960  .8 

1411.0 

1923  .0 

21 

38.39 

76.78 

115.17 

153.56 

191  .95 

383.9 

506.7 

1012.0 

1520.0 

2027.0 

22 

40.40 

80.80 

121.20 

160.60 

202  .00 

404.0 

533.3 

1067.0 

1600.9 

2133.0 

23 

42.45 

84.90 

127.35 

169.80 

212.25 

424.5 

560.3 

1121  .0 

1681  .0 

2241  .0 

24 

44.52 

89.04 

133.56 

178.08 

222.60 

445.2 

587.7 

1175.9 

1763.0 

2351  .0 

25 

46.63 

93.26 

139.89 

186.52 

233.15 

466.3 

615.5 

1231.0 

1847.0 

2462  .0 

26 

48.77 

97.54 

146.31 

195.08 

243.85 

487.7 

643.7 

1287.0 

1931  .0 

2575  .0 

27 

50.95 

101  .90 

152.85 

203.80 

254.75 

509.5 

672.6 

1345.0 

2018.0 

2690  .0 

28 

53.17 

106.34 

159.51 

212.68 

265.85 

531  .7 

701  .8 

1404  .0 

2105  .0 

2807  .0 

29 

55.43 

110.86 

166.29 

221  .72 

277.15 

554.3 

731  .7 

1463.0 

2195.0 

2927  .0 

30 

57.74 

115.48 

173.22 

230.96 

288.70 

577.4 

762.1 

1524  .0 

2286.0 

3048  .0 

LOCATING  DRILL-HOLE  SITES 


105 


USE  OF  TABLE. — Table  XI  may  be  used  in  many  cases  to 
simplify  calculations.  Where  the  dips  are  given,  it  is  a  simple 
matter  to  use  the  table.  In  Fig.  47  the  angle  9  is  25°,  and  the 
distance  is  1320  ft.  or  J^  mile.  From  the  table  one  finds  the 
depths  from  the  horizontal  to  be  615.5  ft.  Add  the  difference  in 
elevation,  E,  or  425  ft.  to  615.5  ft.;  then  615.5  +  425  =  1040.5, 
or  1041  ft.,  approximately. 

Method  2  (See  Fig.  68).— The  second  method  is  the  graphic 
method.  By  using  cross-section  paper  one  accurately  lays  out 


Scale : 


Level 


m 


FIG.  68. — Graphic  method  of  finding  depth. 

the  surface  and  then  the  underground  structure  as  closely  as 
possible.  By  this  method  one  can  approximately  determine  the 
depths  to  certain  formations  and  scale  them  off  directly. 

Where  this  method  is  employed  the  underground  structure 
is  clearly  shown  by  cross-sections.  Accurate  depths  are  only 
possible  where  well  logs  are  accessible,  but  very  good  approxi- 
mations may  be  obtained  by  the  graphic  method.  Many  large 
oil  companies  use  the  graphic  method  of  mapping  their  oil 
territory. 

Method  3  (See  Fig.  69). — Where  no  oil  sands  are  in  evidence 
and  the  anticline  is  closed,  the  only  way  to  determine  depths  is 


106 


PRACTICAL  OIL  GEOLOGY 


by  means  of  some  definite  horizon.     When  this  horizon  is  known 
either  through  fossils  or  lithologic  evidence  as  explained  under 


FIG.  69. — Illustrates  use  of  geological  column. 


FIG.  70. — Shows  curving  of  axis  underground.     (After  Craig.) 


Stratigraphy  (Chapter  II),  the  probable  depth  to  oil  is  obtained 
by  comparing  the  relations  of  the  known  horizon  to  the  other 


LOCATING  DRILL-HOLE  SITES  107 

formations  shown  in  the  geological  column.  Suppose  that  a 
known  horizon  is  found  and  oil  should  supposedly  occur  below 
at  a  certain  depth,  then  add  the  depth,  taken  from  the  geo- 
logical column,  to  the  depth  of  the  known  horizon.  By  such  a 
method,  the  depth  to  oil  may  often  be  approximately  determined. 
Curved  Axes. — In  some  cases  the  axes  of  anticlines  show  a 
tendency  to  overturn  and  to  shift  at  the  top.  As  shown  in  Fig. 
70,  the  axis  of  the  fold  describes  a  curve.  Where  such  is  the 
case,  the  hade  or  slope  of  the  axis  at  the  surface,  represented  by 
the  straight  line,  would  be  misleading.  The  true  condition  is 
the  curved  line.  Such  curved  axes  are  often  found  in  folds  with 
sharp  crests.  Calculations  such  as  the  preceding  may  then  be 
misleading. 


CHAPTER  VII 
FACTORS  IN  OIL-WELL  DRILLING 

Choosing  a  Rig. — When  a  site  has  been  selected  it  is  necessary 
to  choose  the  proper  kind  of  rig  for  testing.  Portable  rigs  or 
permanent  rigs  may  be  used.  If  it  is  desired  to  drill  a  series  of 
holes  from  300  to  1000  ft.  deep,  a  portable  rig  capable  of  drilling 
1200  or  1500  ft.  may  be  employed  for  this  purpose.  Such 
rigs  (see  Fig.  71)  are  capable  of  excellent  work  up  to  1000  ft. 
They  are  admirably  adapted  for  regions  where  the  formations 
require  little  or  no  casing  to  keep  the  drill  holes  in  shape.  How- 
ever, when  it  becomes  necessary  to  handle  several  " strings" 
of  casing,  permanent  rigs  are  much  more  desirable  than  port- 
able. The  portability  of  a  rig  is  only  one  feature  of  rig  selection 
and  has  to  do  chiefly  with  the  convenience  of  moving  a  drilling 
machine.  The  system  of  drilling  most  favorable  to  meet  local 
geological  conditions  must  be  selected.  There  are  three  principal 
systems  employed  in  field  work :  (A)  Standard  cable-tool  system 
(see  Fig.  72);  (B)  hydraulic  rotary  system  (see  Fig.  73),  and 
(C)  combination  of  A  and  B. 

The  choice  of  one  of  these  systems  depends  principally  upon 
geological  factors.  A  discussion  of  these  factors  and  their  effects 
upon  the  drilling  of  holes  gives  a  key  to  the  selection  of  the  rig. 
As  nearly  all  oil  men  are  familiar  with  the  various  drilling  sys- 
tems, descriptions  of  the  same  will  be  passed  over  here. 

The  standard  cable-tool  system  is  perhaps  the  most  generally 
used  and  most  popular  system  at  present.  An  addition,  or, 
rather,  an  improvement  to  this  system  consists  in  what  is  called 
the  "circulator  method."  In  this  method  standard  cable- 
tools  are  employed  while  water  is  caused  to  circulate  through  a 
casing  as  with  the  hydraulic-rotary  system. 

In  drilling  an  oil  well  the  four  chief  objects  in  view  are:  (1) 

108 


FACTORS  IN  OIL-WELL  DRILLING  109 

To  find  commercially  valuable  deposits;  (2)  to  drill  a  favorable 
sized  hole  for  obtaining  and  maintaining  a  good  production; 


FIG.  71. — Portable  drilling  rig. 

(3)  to  drill  in  as  short  a  time  as  possible;  and  (4)  to  exclude 
all  water  from  the  oil  sands. 


110 


PRACTICAL  OIL  GEOLOGY 

D*     Dj 


FIG.  72. — Standard  caBle-tool  drilling  rig. 


FACTORS  IN  OIL-WELL  DRILLING 


111 


112  PRACTICAL  OIL  GEOLOGY 

(1)  LOCATING  has  to  deal  with  the  probabilities  of  finding  oil, 
and  was  discussed  at  length  in  Chapter  VI. 

(2)  FAVORABLE  SIZE  HOLES — (Casing). — The  following  dis- 
cussion of  favorable  sized  hole  applies  more  specifically  to  wells 
in  new  or  "wildcat"  districts.     After  the  first  wells  have  been 
drilled,  it  may  be  and  often  is  found  that  it  is  possible  to  dispense 
with  one  or  more  strings  of  casing  in  drilling  later  wells,  or  start 
with  holes  of  smaller  diameter.     It  is  generally  considered  desir- 
able to  finish  a  hole  with  a  casing,  diameter  not  under  4J^  in., 

^better  sizes  being  6K  to  8Ji  in.  To  obtain  a  400-ft.  hole  with  a 
diameter  of  6J^  in.,  one  would  proceed  as  follows:  To  allow  for 
probable  water  sands,  trouble  with  the  upper  casing,  and  acci- 
dents, it  is  advisable  to  start  with  the  top  diameter  of  10  in.  This 
permits  the  use  of  S^-in.  casing  inside  the  10  in.,  6%-in.  inside 
the  8J^-in.,  and  then  4J£-in.  inside  the  6J^-in.  In  many  cases  it 
will  be  possible  to  finish  a  hole  to  this  depth  with  one  or  two 
strings  of  casing,  10  and  8J^-in.  sizes.  The  two  extra  sizes,  6J4 
and  4J^  in.,  allow  for  unusual  trouble. 

A  standard  cable-tool  hole  2500  ft.  to  4500  ft.  in  depth  would  be 
started  with  16-in.  or  even  18-in.  casing.  This  wouldallow a  12J£- 
in.  inside  the  16-in.,  10-in.  inside  the  12J£-in.,  8J^-in.  inside 
the  10-in.,  6j^-in.  inside  the  SJ^-in.,  and  4J^-in.  inside  the 
6)^-in.  Even  then,  3-in.  casing  might  be  used  inside  the  4J^- 
in.  Such  an  unusual  amount  of  casing  is  seldom  used,  as  holes 
are  generally  finished  with  8J^  or  GJ^-in.  sizes  when  starting 
with  the  16-in.  hole.  The  casing  noted  above  is  necessary  with 
the  standard  cable  system  of  drilling  in  California. 

In  Oklahoma,  Kansas,  Illinois,  .and  the  Eastern  oil  fields,  drill- 
ing practice  varies  from  this  considerably.  On  the  whole,  how- 
ever, the  formations  are  older  and  more  consolidated,  hence 
" stand  up"  better  or  do  not  cave  so  readily,  though  in  certain 
of  the  Oklahoma  oil  fields,  especially  Blackwell  in  Kay  County, 
and  Healdton  in  Carter  County,  conditions  approximating 
California  conditions  occur.  Also  the  sand  conditions  are  well 
understood.  It  is  generally  only  necessary  to  shut  off  the  water 
sands  from  the  oil  sands;  though  in  some  cases,  gas  sands  must 
be  conserved. 


FACTORS  IN  OIL-WELL  DRILLING  113 

It  is  the  rule  to  use  a  conductor  to  shut  off  surf  ace  for  water,  and 
then  drill  until  a  water  sand  is  encountered.  In  drilling  "dry" 
(i.e.,  without  any  water  except  that  put  in  from  the  top)  as  a  rule 
the  water  sand  is  cased  off,  the  hole  is  then  reduced  and  carried 
to  the  oil  sand.  If  a  second  sand  is  encountered  the  same  pro- 
cedure is  followed  or  the  hole  is  underreamed  and  the  casing 
loosed  from  its  first  position  and  set  lower,  if  the  distance  is  not 
too  great.  Where  but  one  water  sand  is  encountered,  wells 
1200-1500  ft.  deep  are  started  with  a  10-in.  conductor  and 
finished  with  a  6J4-in.  hole,  having  only  one  string  of  8^-in. 
casing  in  the  hole  to  shut  off  water,  and  from  10  to  20  ft.  of  10 
in.  casing  as  conductor. 

In  rotary  drilling  the  two  holes  described  above  would  be 
finished  as  follows:  The  400-ft.  hole  would  have  two  strings  of 
casing,  one  10  in.  in  diameter,  the  other  8J^  in.;  2500-ft.  holes 
would  be  finished  with  two  strings  of  casing,  one  12J^  in.  in 
diameter,  the  other  8J4  m->  or  even  10  in.,  though  in  some  unusual 
cases  three  or  four  strings  might  be  required. 

The  principal  factors  involved  in  the  above  selection  of  casing 
are,  namely-,  water  sands  and  quicksands.  In  both  systems  of 
drilling  it  is  necessary  to  make  sure  that  all  water  is  kept  from 
entering  the  oil  sands  from  any  formation  lying  above  the 
sand. 

In  some  cases  one  may  prospect  below  a  proven  oil  sand  and 
encounter  a  lower  water  sand,  and  then  below  that  several  oil 
sands.  It  then  becomes  desirable  to  use  an  extra  string  of  casing 
to  shut  off  the  water  between  the  oil  strata,  though  not  ab- 
solutely necessary  if  there  is  little  water  below. 

In  rotary  drilling,  the  drill  hole  is  kept  full  of  circulating  water. 
Quicksands  are  easily  penetrated  by  this  method  as  the  head  of 
the  water  in  the  hole  keeps  the  sand  from  coming  into  the  hole 
too  freely.  With  standard  cable-tools,  however,  quicksands  or 
loose,  friable  sands  of  any  kind  become  a  source  of  trouble,  and 
require  "  casing  off."  Where  quicksands  are  present  the  standard 
hole  would  require  at  least  one  more  string  of  casing  than  by 
using  the  rotary  system.  However,  where  the  circulator  method 

8 


114 


PRACTICAL  OIL  GEOLOGY 


is  employed  the  rotary  system  has  no  advantage  over  the  standard 
tools  as  to  the  amount  of  casing.  Water  sands  and  quicksands 
must  be  considered  in  another  light.  Even  when  they  do  not 
affect  drilling  they  may,  later,  when  under  considerable  head, 
collapse  the  casing  and  destroy  the  hole.  This  becomes  a  dis- 
tinct evil  and  causes  a  great  deal  of  trouble  and  expense.  To 
overcome  these  pressures  extra  heavy  casings  must  be  used. 

We  have  now  considered  the 
principal  requirements  of  cas- 
ing as  affected  by  geological 
factors.  Discussion  of  the 
above  requirements  apply 
where  drilling  is  carried  on  in 
soft  formations.  Where  lime- 
stones or  sandstones  are  the 
predominant  features,  there 
is  little  need  for  casing  except 
to  hold  back  water  or  prevent 
the  escape  of  oil  into  dry  sands. 
CROOKED  HOLES. — Another 
requirement  for  favorable 
holes  is  that  they  be  straight. 
Crooked  holes  cause  unneces- 
sary expense,  and  later  cause 
great  wear  and  tear  in  the  pro- 
ducing well.  Sometimes  crooked  holes  seriously  interfere  with 
the  entrance  of  casing.  This  results  in  loss  of  time  and  some- 
times loss  of  part  of  the  casing.  The  principal  cause  of  crooked 
holes  is  the  striking  of  hard  strata  that  have  steep  dips.  Sup- 
pose one  is  drilling  in  clay  or  shale,  and  that  the  drill  sud- 
denly strikes  a  hard  stratum  of  sandstone.  (See  Fig.  74.)  If 
the  drill  is  allowed  to  proceed  rapidly,  it  will  follow  down  the 
hard  stratum  until  the  drill-stem  strikes  the  opposite  wall  of 
the  hole.  The  longer  the  drill-stem  the  less  the  deflection.  If 
drilling  proceeds  slowly  a  straight  hole  will  result  without  much 
extra  trouble  or  loss  of  time. 


FIG.  74. — Rotary  bit  following  hard 
stratum. 


FACTORS  IN  OIL-WELL  DRILLING  115 

With  the  rotary  system  the  deflection  of  the  drill  is  limited 
by  the  elasticity  of  the  drill  rod  and  when  the  same  is  bent  to 
strike  the  opposite  wall,  the  drill  will  proceed  through  the  hard 
formation.  Slow,  careful  drilling  will  also,  in  this  case,  give  a 
straight  hole.  When  drilling  through  steep  beds,  it  is  a  wise 
precaution  to  proceed  slowly  when  a  hard  stratum  is  encountered* 
Knowledge  of  such  a  stratum  is  quickly  ascertained  by  any  com- 
petent driller  who  instinctively  learns  to  recognize  the  difference 
of  strata  by  the  way  in  which  the  blow  of  the  drill  is  transmitted 
through  the  drilling  line.  A  good  driller  can  almost  immediately 
tell  by  the  stroke  of  the  tools  when  the  drilling  bit  has  encountered 
a  hard  stratum. 

(3)  TIME  OF  DRILLING. — The  factors  immediately  effective 
in  speed  of  drilling  are: 

(1)  Depth,  (2)  diameter  of  hole  desired,  (3)  water  sands,  (4) 
quicksands,  (5)  dry  sands,  (6)  hardness  of  strata,  (7)  boulders, 
and  (8)  cavities. 

Necessarily  the  length  of  time  taken  to  lower  and  raise  the 
tools  increases  directly  with  the  depth.  Also  to  handle  long 
strings  of  casing  more  effectively  and  quickly,  it  becomes  neces- 
sary as  a  time-saving  element  to  erect  high  derricks  in  which 
three  or  four  lengths  of  casing,  each  length  being  20  to  22  ft. 
over  all,  can  be  stood  in  the  derrick  without  trouble. 

Such  stands  are  from  60  to  80  ft.  in  length.  To  pull  these 
stands,  it  is  necessary  to  have  derricks  that  are  from  82  ft.  to 
120  ft.  in  height.  High  derricks  are  employed  injholes  over  1500 
ft.  in  depth.  The  following  figures  will  give  some  idea  of  the 
heights  of  derricks  used  for  varying  depths.  For  shallow  holes, 
400  to  600  ft.,  where  the  casing  may  be  rapidly  handled,  64-ft. 
frame  derricks  would  be  used.  Up  to  1500  ft.,  a  72-ft.  derrick 
would  be  used;  up  to  3000  ft.,  an  82  to  90-ft.  derrick  and  beyond 
that  depth,  106  to  120-ft.  In  rotary  drilling,  82  to  120-ft.  der- 
ricks are  used,  the  82-ft.  being  used  up  to  2000  ft,,  and  higher 
derricks  above  that  depth. 

The  length  of  cable  is  also  directly  affected  by  depth.  The 
greater  the  depth,  the  longer  the  cable  becomes  and  the  larger  its 
diameter  within  certain  limits. 


116  PRACTICAL  OIL  GEOLOGY 

The  size  of  drilling  cables  varies  with  the  diameter  of  the  hole 
and  the  depth.  For  deep  holes,  2000  to  3500  ft.,  with  large  di- 
ameters, 8%  to  6J^-in.  casings,  cables  1  to  \Y±  in.  in  diameter 
are  used.  For  holes  up  to  2000  ft.,  %  to  %-in.  lines  are  used. 
Deep  holes  3  in.  in  diameter  can  effectively  use  %-in.  lines. 

In  drilling  with  a  standard  rig  the  best  speed  is  made  in  a 
dry  hole.  In  oil-field  phraseology  a  "dry  hole"  is  one  in  which 
water  must  be  put  in  the  hole  to  thin  the  drill  cuttings,  to  keep 
the  bit  from  overheating,  and  to  soften  the  formation.  When  a 
water  sand  is  encountered,  the  hole  becomes  a  "wet"  one, 
that  is,  furnishes  water  enough  to  meet  the  needs  of  drilling.  In 
"dry"  holes  it  is  only  necessary  to  keep  a  small  quantity  of 
water  in  the  hole;  "wet"  holes  are  nearly  full  and  the  surplus 
water  retards  the  stroke  of  the  tools  and  weakens  their  impact, 
resulting  in  slower  drilling. 

In  many  fields  Manila  cables  are  used  until  water  is  struck, 
and  from  there  on  steel  cables  are  employed.  In  "wet"  holes 
water  retards  the  speed  of  drilling  to  such  an  extent  that  it 
becomes  necessary  either  to  case  off  the  water  sands  or  use  wire 
cables.  Wire  ropes  or  cables  are  commonly  employed  with 
standard  cable-tools  for  deep  drilling,  2000  ft.  or  more.  With 
the '  hydraulic-rotary  system,  however,  water  does  not  retard 
the  speed  of  drilling;  indeed  the  extra  water  saves  the  introduc- 
tion of  water  from  the  surface,  and  thus  becomes  an  important 
factor  in  economical  drilling.  Where,  however,  the  flow  of  water 
is  very  strong,  the  rotary  mud  is  washed  from  the  hole  and  slow 
progress  results. 

Where  the  head  of  water  in  the  bore  hole  is  greater  than  that 
in  the  water  sands,  circulation  is  affected,  and  the  water  instead 
of  returning  to  the  surface  enters  the  water  sand.  To  overcome 
this  trouble,  it  is  necessary  to  pump  mud  into  the  hole.  This 
mud  fills  the  interstices  of  the  sand  and  keeps  the  water  from 
escaping  from  the  bore  hole. 

Dry  sands  are  pernicious  in  both  systems  of  drilling.  These 
Sands  absorb  water  and  cause  an  undue  "quantity  to  be  pumped 
into  the  hole,  since  without  water  the  bits  would  soon  overheat, 


FACTORS  IN  OIL-WELL  DRILLING 


117 


and  the  cuttings  would  accumulate  to  such  an  extent  that  they 
would  retard  the  action  of  the  drill.  By  introducing  mud  into 
the  hole  the  walls  of  the  dry  sands  are  puddled  and  the  water 
held  in  the  hole.  It  is  often  necessary  to  put  in  many  tons  of 
mud  which  is  mixed  in  sump  holes  at  the  sur- 
face. This  mud  has  the  consistency  of  thick 
cream  and  is  either  poured  or  pumped  into  the 
hole. 

Hardness  of  Strata  and  Its  Effect  upon  Drilling 
Operations. — 1.  SANDS. — Drilling  in  sands  is  gen- 
erally fast  work,  especially  where  the  rotary  or 
"circulator"  systems  are  employed.  Standard 
cable-tools  penetrate  sands  rapidly  though  the 
bits  are  worn  quickly  by  the  sharp  sand  grains. 
Where  the  sands  have  a  tendency  to  shift  or 
cave,  standard  cable-tools  have  a  hard  time  to 
"make  hole."  In  such  cases,  days  are  some- 
times required  to  drill  even  a  few  feet.  Under 
such  conditions,  rotary  sand  circulators  do  the 
best  work."  With  them  it  is  possible  to  drill 
from  100  to  200  ft.  per  24  hours  in  sandy  strata. 

2.  SANDSTONES. — Hard  sandstones  and  lime- 
stones are  quickly  shattered  by  the  heavy  blows 
of  the  standard  cable-tools.  The  heavy  standard 
bit  (see  Fig.  75)  makes  good  progress  in  such 
formations,  though  the  wear  and  tear  on  the  bits 
is  fairly  great.  In  rotary  drilling,  however, 
especially  where  a  fish-tail  bit  (see  Fig.  76)  is 

employed,  little  headway  is  made,  and  in  such  c 

Standard  cable- 
cases   another   form    of   bit   must  be  employed,  tool  drill-bit. 

either  a  core  barrel,  adamantine  with  a  reversed 

fish-tail  bit,  a  rotary  shoe  with  adamantine  or  the  Sharp   & 

Hughes  bit.     (See  Fig.  77.) 

In  the  writer's  estimation,  the  early  lack  of  success  of  the 
rotary  system  of  California  was  due  most  of  all  to  the  drilling 
of  the  shell  or  sandstone  formations  with  fish-tail  bits.  Only  a 


7-~ 


118 


PRACTICAL  OIL  GEOLOGY 


few  satisfactory  holes  had  been  put  down  with  a  rotary  and 
these  required  a  longer  time  than  seemed  necessary.  The  fish- 
tail bit  was  almost  invariably  employed  in  drilling  through  all 
formations;  though  at  present  it  excels  for  drilling  in  soft  ma- 
terials, its  use  is  deprecated  in  hard  formations. 

Another  trouble  with  sandstones,  especially  where  a  string 
of  casing  is  carried  close  to  the  bottom  of  the  hole,  lies  in  the 
fact  that  the  drill  which  fits  snugly  inside  the  casing  does  not 
make  a  large  enough  hole  in  the  sandstone  to  allow  the  casing 


FIG.  76.— Fish-tail  ro- 
tary bit. 


FIG.  77.— Sharp  &  Hughes 
hard  rock  rotary  bit. 


FIG.  78.— Un- 
derreamer. 


to  go  through  it.  To  meet  this  objection,  underreamers  are 
used.  These  underreamers  (see  Fig.  78)  are  really  expansion 
bits.  When  let  down  through  the  casing,  these  bits  project  be- 
yond the  casing  shoe  and  enlarge  the  diameter  of  the  hole  so  that 
the  casing  can  be  forced  downward. 

3.  CLAYS  AND  SHALES. — In  drilling  through  clays  and  shale 
with  standard  cable  tools,  the  greatest  difficulty  encountered 
is  the  tendency  of  the  clay  to  stick  to  the  bit.  This  tendency 
makes  drilling  in  the  soft  clay  a  slow  process.  There  is  also  the 
tendency  of  the  clay  to  "creep"  or  to  cave  into  the  hole.  This 


FACTORS  IN  OIL-WELL  DRILLING  119 

tendency  may  be  overcome  to  some  extent  by  keeping  the  hole 
full  of  water,  and  by  carrying  the  casing  close  to  the  drill.  The 
hydraulic-rotary  system  and  the  " circulator"  method  are 
well  adapted  for  drilling  through  clay  and  shale.  By  these 
methods  one  literally  washes  out  the  soft  material  and  can  make 
very  rapid  progress,  in  exceptional  cases  from  200  to  300  ft.  having 
been  made  in  24  hours.  Clay  does  not  dull  bits  rapidly  so  that 
when  drilling  in  it  there  is  little  need  to  draw  the  tools  from  the 
hole  to  sharpen  them. 

4.  BOULDERS. — Boulders  are  a  source  of  trouble  wherever 
found.  They  may  occur  as  concretions  in  sands  and  shales,  or 
as  conglomerate  beds  made  up  of  material  ranging  in  size  from 
that  of  an  egg  to  the  size  of  one's  head  and  larger. 

Concretions  are  encountered  in  many  regions  and  explain  the 
presence  of  unusually  hard  streaks  in  what  would  otherwise  be 
solid  beds  of  shale  and  sand.  Concretions  are  harder  than  the 
surrounding  material  and  thus  check  the  speed  of  drilling  some- 
what. In  a  few  cases,  however,  large  boulders  may  be  mistaken 
for  solid  beds  of  sandstones  and  casing  landed  on  them. 

Where  -the  boulders  consist  of  material  different  from  the 
immediate  formation,  one  is  warned  against  landing  the  casing 
on  them.  The  greatest  trouble  with  boulders,  however,  comes 
from  the  boulder  beds.  When  a  drill  has  passed  through  such 
beds  there  is  always  a  liability  that  a  boulder  may  fall  in  above 
the  drill-bit  and  wedge  if  fast.  Especially  is  this  the  case  in  rotary 
drilling  where  a  fish-tail  bit  is  used.  In  this  case  the  boulders 
lodge  above  the  bit  between  the  drill  rods  and  the  wall  of 
the  hole  and  wedge  the  bit  tight.  Many  hoes  are  thus  lost. 
It  is  sometimes  possible  to  unscrew  the  rotary  pipe  and  later 
drill  up  the  rotary  bit  or  sidetrack  the  same,  if  the  walls  of 
the  hole  are  of  clay  or  shale.  Where,  however,  the  walls  are 
sandstone  or  limestone,  sidetracking  is  out  of  the  question.  In 
some  cases  it  is  possible  to  break  up  the  boulders  by  letting  down 
a  string  of  light  cable  tools  alongside  the  rotary  pipe. 

One  ingenious  operator  in  a  case  where  the  driller  could  neither 
drill  nor  pull  the  pipe,  devised  a  method  that  proved  effectual. 


120  PRACTICAL  OIL  GEOLOGY 

Oil  sand  was  expected  within  a  few  feet.  To  abandon  the  hole 
meant  a  heavy  monetary  loss.  As  the  drill  pipe  was  4^  in. 
in  diameter  the  operator  determined  to  drill  through  the  rotary 
bit  below  by  using  a  steel  bit  harder  than  the  rotary  bit.  Using 
2-in.  drill  rods  he  milled  through  the  rotary  bit  below  and  reached 
oil  sand,  obtaining  a  good  flow  of  oil  through  a  2-in.  hole. 

5.  CAVITIES. — Cavities  are  very  uncertain  factors  in  oil  well 
drilling.  They  may  be  classified  as  follows: 

(1)  Those  along  fault  or  fracture  planes;  (2)  solution  cavities; 
and  (3)  " washout"  cavities. 

In  some  regions  like  Boulder,  Colo.,  cavities  have  been  reported 
in  drill  holes.  They  are  supposed  to  be  in  fracture  plane.  In 
these  regions  the  drill  tools  drop  suddenly  into  underground 
fractures  or  crevices  due  either  to  faulting  or  folding.  Such  cases 
are  assumptive  and  explain  the  unusual  phenomena  better  than 
anything  else. 

In  limestone  regions  solution  cavities  are  plentiful.  Indeed, 
great  underground  caverns  are  of  frequent  occurrence.  In  such 
regions  the  disappearance  of  water  in  a  drill  hole  and  the  sudden 
dropping  of  tools  is  in  no  way  a  mystery,  but  rather  to  be 
expected  at  any  time. 

"Washout"  cavities  are  purely  artificial  and  are  due  to  the 
withdrawal  of  soft  sand  or  shale  from  under  a  hard  stratum  of 
sandstone  or  limestone. 

In  drilling,  the  softer  sand  may  cave  and  cause  a  cavity  to 
form  below  a  hard  stratum.  If  a  hydraulic  rotary  be  used,  great 
quantities  of  sand  may  be  washed  from  the  hole.  Where  the 
sand  begins  to  wash  out,  especially  where  there  is  any  tendency 
to  shift,  a  sudden  rush  of  sand  may  collapse  the  casing  or  perhaps 
cover  up  the  drill  tools,  causing  a  difficult  fishing  job.  In  one 
instance  a  cavity  of  this  kind  was  large  enough  to  hold  seven  or 
eight  wagon  loads  of  boulders  and  brickbats,  and  over  2000  ft. 
of  old  bull-rope  and  Manila  line  was  required  before  the  cavity 
could  be  filled  enough  to  form  a  bridge,  upon  which  it  was  desired 
to  rest  a  string  of  casing. 

Rock  Pressure. — Drillers  and  operators  often  speak  of  "rock 


FACTORS  IN  OIL-WELL  DRILLING 


121 


pressure"  as  the  cause  of  gas  pressure.  However,  the  writer  pre- 
sents another  aspect  of  rock  pressure  and  its  practical  application 
to  caving  and  heaving  troubles — also  the  collapsing  of  casing 
which  is  notably  the  trouble  in  California,  where  the  sediments 
are  not  as  compact  as  in  Pennsylvania  and  the  Mid-Continent. 
Compression  Due  to  Rock  Pressure. — Where  the  earth's  strata 
are  thrown  into  folds,  these  folds  are  under  compression  due  to 
gravitative  influence  or  due  to  the  weight  of  the  overlying  forma- 
tions. Some  strata  can  resist  less  than  others  and  are  more  sub- 
ject to  movements  than  others.  Thus  soft  sands  and  shales  are 
more  subject  to  change  due  to  compressive  action  than  are 
sandstones  and  limestones.  Shales  and  sands  creep,  shift  or 
heave  under  compressive  power  that  would  not  noticeably  affect 
the  harder  formations. 


FIG.  79. — Illustrates  direction  of  resultant  L,  which  tends  to  collapse  casing. 


Folds  furnish  excellent  conditions  for  unbalanced  pressures 
and  consequently,  when  drill  holes  which  disturb  the  equilibrium 
of  a  series  of  pressures  are  put  down,  collapsing  of  the  casing  or 
caving  of  the  open  hole  results. 

Consider  the  fold  shown  in  Fig.  79.  At  any  point  the  vertical 
pressure  of  the  strata  may  be  resolved  into  a  component  at  right 
angles  to  the  dip  of  the  strata.  This  component  R  can  be  resolved 
into  a  resultant  L,  which  we  will  call  the  lateral  pressure.  It  is 
the  action  of  this  lateral  pressure  which  the  writer  believes 
explains  casing  troubles  more  fully  than  any  other  theory.  The 


122  PRACTICAL  OIL  GEOLOGY 

resultant  toward  the  syncline  compresses  casing  in  holes  which 
are  drilled  down  the  dip  from  the  outcroppings  of  oil  strata. 

By  trigonometry  the  resultant  compressive  power  L  is  given  by 
the  following  expression: 

L  =  cos  angle  of  dip  X  cos  90  —  angle  of  dip  W. 
(W  is  the  weight  of  overlying  strata.) 

Theoretically  this  pressure  is  greatest  when  the  angle  of  dip  is 
45°,  decreasing  when  the  angle  decreases  or  increases  above  45°. 

Measure  of  Pressure. — Suppose  a  thickness  of  strata  of  3000 
ft.  composed  of  clays,  shales,  conglomerates,  sands,  and  sand- 
stones. The  average  specific  gravity  of  such  strata  is  2.5.  That 
is,  1  cu.  ft.  of  such  material  weighs  2J^  times  the  weight  of  a  cubic 
foot  of  water  under  atmospheric  pressure  and  at  60°F. 

Water  weighs  62.5  Ib.  per  cu.  ft.  Then  1  cu.  ft.  of  the  above 
materials  will  weigh  62.5  X  2.5  =  156  +  Ib. 

A  column  of  stone  1  ft.  high  and  1  sq.  in.  at  the  base  will  exert 

i 1\(\ 
a  pressure  :rrr  =  1-08  Ib.  per  sq.  in. 

A  column  of  strata  3000  ft.  thick  will  exert  a  pressure  of  1.08 
X  3000  =  3240  Ib.  per  sq.  in. 

The  lateral  resultant  of  this  for  an  angle  of  dip  of  45°  will  be  L 
=  cos  45  X  (cos  90  -  45)  X  3240;  or  0.5  X  3240  =  1620. 

For  an  angle  of  dip  of  30°L  =  cos  30  +  cos  - '°  ~  3°^  X  3240 
=  0.866  X  0.5  X  3240  =  1403. 

(Qf)    ()(\\ 

Likewise  for  60°  dip  L  =  cos  60  X  cos  -  ^p  -  X  3240  = 

0.5  X  0.866  X  3240  =  1403. 

Such  high  pressures  are  certainly  great  enough  to  cause  casing 
collapses  in  weak  or  defective  casings.  Even  at  a  few  hundred 
feet,  pressures  of  300  to  400  Ib.  may  cause  the  formations  to  cave 
or  to  creep  into  the  drill  hole. 

The  deeper  the  hole  is  carried,  the  greater  the  pressures,  and  the 
greater  the  need  for  heavy  casing.  Table  XII  gives  some  casings 
tests.  Note  that  the  larger  the  casing  and  the  less  the  weight,  the 


FACTORS  IN  OIL-WELL  DRILLING 


123 


less  resistance  it  offers  to  compression.  In  all  engineering  work  it 
is  usual  to  use  factors  of  safety  to  insure  sufficient  strength  to 
make  up  for  any  weakness  in  iron,  steel,  stone,  or  other  materials 
due  to  flaws  in  the  material.  Such  factors  in  oil-field  work  are  2 
and  3.  A  factor  of  safety  of  2  means  that  a  casing  has  twice  the 
resistive  strength  necessary  to  offset  collapsing  pressures.  A 
12l^-in.  casing  weighing  40  Ib.  per  ft.  would  withstand  but  402 
Ib.  lateral  compression. 


TABLE  XII. — SHOWING   COLLAPSING   PRESSURES   OR  LAP-WELDED   STEEL 
CASING  FOR  SIZES  COMMONLY  USED  IN  CALIFORNIA 


Size,  in. 

Weight 
per 
ft.,  Ib. 

Inside 
diameter, 
in. 

Outside 
diameter, 
in. 

Thickness, 
in. 

Collapsing 
pressure, 
Ib.  per 
sq.  in. 

Equiva- 
lent water 
column, 
ft. 

Water 
column, 
factor  of 
safety  2,  ft. 

4H 

15.0 

4.500 

5.000 

0.250 

2944 

6790 

3395 

fifi 

20.0 

5.370 

6.000 

0.315 

3160 

7280 

3640 

6H 

20.0 

6.000 

6.625 

0.312 

2704 

6230 

3115 

26.0 

5.845 

6.625 

0.390 

3717 

8560 

4280 

2§rO 

5.775 

6.625 

0.425 

4167 

9600 

4800 

6H 

20.0 

6.437 

7.000 

0.281 

2096 

4830 

2415 

26.0 

6.312 

7.000 

0.344 

2867 

6600 

3300 

28.0 

6.220 

7.000 

0.390 

3440 

7930 

3965 

7* 

26.0 

7.390 

8.000 

0.305 

1914 

4410 

2205 

SM 

28.0 

8.015 

8.625 

0.305 

1680 

3870 

1935 

32.0 

7.935 

8.625 

0.345 

2080 

4790 

2395 

36.0 

7.875 

8.625 

0.375 

2383 

5490 

2745 

38.0 

7.765 

8.825 

0.430 

2928 

6750 

3375 

43.0 

7.625 

8.625 

0.500 

3638 

8380 

4190 

ta 

33.0 

9.500 

10.000 

0.250 

780 

1800 

900 

10 

40.0 

10.000 

10.750 

0.375 

1638 

3770 

1885 

48.0 

9.850 

10.750 

0.450 

2234 

5150 

2575 

64.0 

9.750 

10.750 

0.500 

2643 

6090 

3045 

UH 

40.0 

11.437 

12.000 

0.281 

641 

1475 

737 

12w 

40.0 

12.500 

13  .  000 

0.250 

402 

927 

463 

45.0 

12.360 

13.000 

0.320 

745 

1717 

858 

50.0 

12.250 

13.000 

0.375 

1109 

2560 

1280 

ia* 

50.0 

13.250 

14.000 

0.375 

936 

2160 

1080 

1SH 

51.3 

15.416 

16.000 

0.292 

314 

724 

362 

124  PRACTICAL  OIL  GEOLOGY 

However,  a  weak  place  in  the  casing  might  fall  below  that  limit 
to  one-half  of  the  normal  strength.  Then  if  pressures  of  300  to 
500  Ib.  are  exerted,  the  casing  may  collapse.  The  remedy  here  is 
simply  to  use  a  casing  sufficiently  heavy  to  have  a  factor  of  safety 
of  2  or  3  if  possible  and  to  use  only  good  casing. 

An  8^-in.  28-lb.  casing  has  a  collapsing  pressure  of  1680  Ib.  A 
lateral  rock  pressure  of  1000  Ib.  may  readily  be  obtained  at  a 
depth  of  2000  to  2500  ft.  The  factor  of  safety  here  is  but  1.68,  so 
that  any  weakness  in  pipe  would  lower  the  compressive  resistance 
to  a  very  dangerous  point. 

So  far  nothing  has  been  said  about  drilling  water  in  a  test  hole  or 
oil  in  a  pumping  well.  Of  course  where  the  hole  is  full  of  water, 
each  1000  ft.  of  water  exerts  a  pressure  of  434  Ib.  per  sq.  in.  inside 
the  casing.  This  pressure  will  help  to  balance  the  lateral  rock 
pressure.  Where,  however,  the  well  is  bailed 'or  pumped  down, 
clanger  threatens. 

Shooting  Wells. — It  is  common  practice  in  the  Pennsylvania 
and  the  Mid-Continent  oil-fields  to  shoot  or  dynamite  the  wells 
after  they  have  been  drilled  into  the  oil  stratum.  This  is  done 
to  break  up  the  formation  so  it  will  form  a  reservoir  for  oil,  and 
also  cause  channels  to  form  in  the  shattered  rock.  Where  the 
formations  are  hard  sandstone,  limestone,  or  very  hard  shale, 
shooting  will  break  up  the  material  sufficiently  to  form  good 
reservoirs  and  channels;  where  the  sands  are  soft  and  flowing,  as 
in  some  of,  the  Calif ornian  and  .Russian  fields,  dynamiting  has 
no  effect.  Where  soft  shale  makes  up  the  oil  stratum,  the  tend- 
ency is  to  compress  or  tighten  the  shales,  thus  keeping  back  the 
oil,  instead  of  allowing  it  to  enter  the  well. 

In  some  instances  especially  where  the  sandstone  is  hard  and 
compact,  too  heavy  a  shooting  charge  of  nitroglycerin  is  used,  and 
as  a  result  the  sandstone  is  pulverized  instead  of  being  made  more 
coarse.  The  pulverized  material  cannot  be  entirely  cleaned  out, 
and  is  liable  to  clog  the  pores  of  the  sandstones  and  keep  back 
production. 

Drilling  through  Coal  Beds. — In  certain  parts  of  Pennsylvania 
and  Illinois,  workable  coal  beds  are  found  above  the  petroleum 


FACTORS  IN  OIL-WELL  DRILLING  125 

formations.  Drill  holes  pass  through  these  coal  beds  before 
entering  the  oil  sands.  Where  the  coal  beds  are  worked  there  is 
danger  of  injuring  the  oil  wells,  also  danger  of  accidents  in  the 
mine,  due  to  escaping  gas  and  injury  from  drilling  accidents. 

A  knowledge  of  such  beds  is  essential  before  commencing 
work,  as  the  coal-mine  owners  must  allow  pillars  of  sufficient 
size  to  protect  the  oil  well.  The  United  States  Bureau  of  Mines 
is  doing  excellent  work  along  this  line  in  Pennsylvania. 

Logs. — (Records). — The  value  of  logs  in  drilling  must  not  be 
underestimated.  Every  company  should  keep  a  close  accurate 
log  of  the  formations  through  which  the  well  or  wells  are  drilled. 
Many  thousands  of  dollars  are  annually  lost  through  not  knowing 
the  location  of  water  or  of  oil  sands. 

When  casing  troubles  are  at  hand  in  oil  wells,  it  is  extremely 
valuable  to  know  the  location  of  important  sands  to  intelligently 
redrill  the  well.  The  knowledge  of  certain  formations  in  a  new 
well  enables  an  operator  to  proceed  much  more  rapidly  with  the 
holes  that  follow.  Contractors  especially  find  such  information 
very  valuable. 

By  far  ioo  little  attention  is  paid  to  logs.  Some  uniform 
system  of  keeping  logs  should  be  in  vogue.  Also  every  oil  man 
should  clearly  understand  the  meaning  of  the  various  terms,  clay, 
shale,  gravel,  etc.  One  driller  calls  a  brown  shale  blue,  or  a 
blue  shale  brown,  when  others  call  it  blue.  This  is  a  needless 
classification.  Blue  and  brown  are  entirely  different  colors,  but 
drillers  sometimes  classify  these  formations  alike.  Then,  too, 
a  sand  and  a  sandstone  are  very  different,  as  also  are  sand,  and 
shale.  Sand  is  very  gritty;  shale  is  softer  and  less  gritty.  Clay 
sticks  to  a  bit  and  unlike  shale  has  no  small  seams  or  layers  in  it. 

One  point  on  which  the  drillers  make  mistakes  is  in  classifying 
the  material  while  wet.  Gray  material  when  wet  is  blue,  and 
brown  and  bluish  shales  when  wet  are  nearly  black.  By  letting 
the  material  dry,  the  true  color  will  appear.  If  one  driller 
classifies  while  the  material  is  dry  and  the  other  while  the  material 
is  wet,  conflicts  will  arise. 

Geologists  generally  differ  little  in  their  classification  of  sands 


126 


PRACTICAL  OIL  GEOLOGY 


and  shales.  If  drillers  and  superintendents  would  only  fol- 
low the  geologist's  classifications,  logs  would  be  much  simplified. 
Anyone  who  has  attempted  redrilling  on  a  well  where  a  number  of 
drillers  have  worked  will  realize  the  utter  folly  of  allowing  half 
a  dozen  different  classifications. 

If  the  logs  are  worth  keeping,  and  they  are  without  doubt 
invaluable,   keep   them  so   carefully  that   no   one   can   mistake 


DAILY  TOUR  REPORT 

Company 

Leas* 

Well  Mo 

Day 

Date 

Tour 

Morning 

Afternoon 

Formations 

Depth 

mation 

Color 

Tezture 

Hardness 

Contents 

Remarks 

From 

To 

Foi 

1 

3 
J 

1 

1 

£ 

1 

1 
1 

a 

I 

Water 

OM  9 

91                , 

t 

a 

r  — 
[I 

hi 

{     1 

a 

=="  —  - 

f 

Casing                                               Time 

MM 

Size 

Depth 

Remarks 

Driller 

Hours 

Tool 
Dressers 

Hours 

1 

—  -  L 

FIG.  80.— Log  form.     (After  Phelps.) 

their  meaning.  How  many  oil  men  today  can  feel  certain  that 
the  logs  are  carefully  kept?  "Wild-cat"  drillers  are  often  crim- 
inal in  their  negligence  and  later,  when  exact  information  is 
desired,  none  is  available. 

Any  careful  classification  will  do  as  long  as  it  is  adhered  to. 
A  form  for  keeping  logs  is  given  above.     (See  Fig.  80.) 

WELL  LOGS  AND  THEIR  INTERPRETATION 

In  the  hurry  and  bustle  of  their  oil-field  work,  the  operators  too 
often  fail  to  take  time  to  study  their  well  logs,  with  the  results 


FACTORS  IN  OIL-WELL  DRILLING 


127 


that  they  obtain  only  part  of  the  benefit  that  they  might  other- 
wise obtain. 

The  following  discussion  is  presented  to  indicate  a  few  points 
that  have  an  important  bearing  on  oil  problems,  but  which,  too 


Scale 
-800 

-700 

-UOO 

-500 

-400 

-300 

-200 

-100 

-0 

-100 

-200 

-300 

-100 

-500 
-000 
-700 
-800 


P2p" 
1                                                     ..--""" 

EC 

T?o 

ZE: 

730        ^" 

XI 

Vao 

n 

780 

650 

640         ^^ 

"^C~" 

ss"~^^^ 

—  r 

^ 

SS^ 

^   -*-'^' 

"840 

So 

'ss 

n 

"GOO 

TTTT 

^^ 

^F^* 

^^ 

"ss 

88 

_.—  - 

s 

AC 

B8 

s 

M 

A 

H 

IT 

ro 

g 

&-| 

380 

1 

LSJ 



-rr 

<~           625 

' 

31 

"LS 

cc: 

LS 

1 

XF 

, 

g 

LS 

52) 

^ 

s 

CLS 

S 

LS          — 

I 

<L* 

150 
) 

( 

^^ 

^ 

170 

^** 

ssZ 

s 

s 

.  ~~  "  ~~~~ 

LS 

s 

i^ 

1 

SS     " 

T-v 

SS 

Wat 

9 

Mean 

Sea 

^Level 

Water 

Wate 

c  > 

§ 

* 

LS 

• 

*'*' 
"Oil          730 

I 

SS 
Water 

= 

— 

XT 

= 

u 

250 

/' 

• 

oil 
ss 

SS 
Water^ 

EX 

^s^ 

— 

860 

ES 

/ 

Water 

' 

s 

^" 

'ss 

D 

11 

vg?^ 

§ 

rr-r 

g 

520 

Z?  i 

^T 

SS 

Water  ' 

J?: 



s 

—.  

s 

-~*"~ 

1 

**"^-^ 

* 

| 

Water 

_^-> 

° 

/ 

S 

s 

S 

"""""" 

s 

^^ 

i 

g 

5 

s 

~~I 

jf'     "" 

• 

bi-L 

5E 

EE 

^ 

E 

"ss 

Water 

"s's- 

Water 

SS 
Water 

,•;-•• 

SS 
Water 

SS  "^*^ 

Water     ' 

If 

ss"" 

Water 

S8j 

Wat 

i 
.F 
» 

-—  j^rmi^r~  — 



'^^ 

:°-^~ 

^~^^ 

F 

8 

7 

6                5 

4 

3 

2 

1 

FIG.  81. — Interpretation  of  well  logs. 


often,  are  overlooked. 

Fig.   81   is  purely  diagrammatic,  presenting,  however,  condi- 


128  PRACTICAL  OIL  GEOLOGY 

tions  known  to  occur  in  several  fields.  At  the  top  of  the  figure 
the  surface  relation  of  the  wells  are  given,  also  the  surface  appear- 
ance of  the  rocks. 

The  wells  are  numbered  1  to  8  across  a  distance  of  5  miles. 
The  elevations  above  sea  level,  obtained  by  a  survey,  are  also 
given.  Below  the  wells  are  skeleton  or  partial  logs  which  are 
placed  in  their  proper  relation  to  sea  level.  The  logs  are  all  on  the 
same  scale. 

Considering  C  as  the  key  horizon,  or  base  bed,  the  other  beds 
A,  Bj  D,  and  E  are  related  to  it.  The  interval  between  the  tops 
of  the  various  beds  and  their  relations  to  the  top  of  Bed  C  are 
found  from  the  well  logs.  For  example,  at  the  top  of  A  is  380 
ft.  above  C,  the  top  of  D,  150  ft.  below  C,  and  the  top  of  E, 
860  ft.  below  the  top  of  C. 

With  these  points  clearly  in  mind  we  can  discuss  the  findings 
from  the  well  logs. 

The  importance  of  well-kept  records  must  not  be  underesti- 
mated, as  accurate  conclusions  cannot  be  obtained  upon  inaccu- 
rate data. 

The  following  structural  conditions  are  shown  in  the  diagram : 

1.  A  synclinal  condition  occurs  near  2,  as  the  beds  at  2  are 
lower  than  at  1  and  3.     A  faulted  condition  might  exist,  but 
such  a  fault  would  act  as  syncline ;  for  all  practical  purposes  the 
result  would  be  the  same. 

2.  An  anticlinal  condition  occurs  near  3  and  between  2  and  4. 
Hole  3  shows  the  beds  higher  than  at  2  and  4,  indicating  an  anti- 
clinal condition. 

3.  A  terrace  occurs  at  5  and  6.     The  holes  at  5  and  6  show  a 
flattening  of  the  dip  giving  a  terrace  condition  there. 

4.  The  average  surface    dip  to  the   west  is  36  ft.  per  mile. 
This  dip  to  the  west  is  obtained  by  subtracting  the  difference  in 
elevations  between  two  points  and  dividing  by  the  miles. 

The  dip  between  1  and  8,  taken  on  the  limestone  shown  at  the 
surface  is  (780  -600)  -f-  5  =  36  ft. 

Again,  the  underground  dip  on  C  shows  as  follows:  The  ele- 
vation of  C  above  sea  level  at  1  is  280  ft.;  at  8  it  is  —50,  or  50 


FACTORS  IN  OIL-WELL  DRILLING  129 

ft.  below  sea  level,  or  a  total  difference  of  330  ft.,  between  the 
two  points.  The  dip  of  the  bed  is  then  330  -5-  5  =  66  ft.  per 
mile. 

5.  The  dip  is  not  constant,  but  varies  from  point  to  point.     As 
is  apparent,  the  dip  from  8  to  6  is  greater  than  at  6  to  5,  or  4 
to  2. 

6.  The  limestone  B  decreases  in  thickness  to  the  West  and  dis- 
appears.    At  1  the  limestone  is  thickest;  at  7  it  disappears. 

7.  The  intervals    between  A  and  (7,  and  C  and  D  increase 
westward.     At  1  the  interval  between  A  and  C  is  380  ft.;  at  8 
the  interval  is  520,  a  thickening  of  520  —  380  or  140  ft.,  in  5 
miles,  or  28  ft.  per  mile.     The  interval  between  C  and  D  at  1 
is  100  ft.  in  5  miles  or  20  ft.  per  mile. 

8.  The  interval  between  C  and  E  increases  eastward  showing 
an  unconformity.     The  interval  between  C  and  E  is  520  ft.  at  8, 
and  860  at  1,  a  difference  of  860  -  520  =  340,  or  a  thickening  of 
68  ft.  per  mile,  east.     Also  the  beds  appear  above  E  at  1,  2,  3,  4 
and  5  that  do  not  appear  at  8.     An  angular  unconformity  is 
plainly  apparent. 

Note  als£  that  the  underground  folded  structure  at  6  and  3  is 
still  strongly  pronounced. 

9.  The  beds  are  in  themselves  variable,  as  is  illustrated  in  F. 
At  1,  the  sand  is  thin  and  increasing  in  thickness  to  3,  thinning  at 
4,  and  thickening  at  5  to  thin  again  at  7. 

10.  The  well  at  6  was  not  drilled  to  the  deeper  sands  at  E  and 
F.     However,  at  3,  the  deeper  sands  have  proven  productive. 
Also,  the  shallow  sand  at  6  has  production,  and  it  is  likely  that 
the  deeper  sands  at  6  will  also  prove  productive,  as  there  is  a  well 
defined  folded  condition  at  that  point,  which  is  favorable  for  an 
oil  accumulation. 

11.  Needless  to  say  the  wells  in  the  upper  sand  rapidly  deepen 
westward. 

12.  As  will  be  noted  there  is  a  distinct  relation  between  the 
underground  and  the  surface  folding.     In  many  cases  surface 
exposures  check  closely,  but  in  some  cases  no  exposures  are  found 
and  the  structure  depends  solely  upon  the  well  log  records. 


130  PRACTICAL  OIL  GEOLOGY 

In  conclusion,  it  is  important  to  reiterate  that  accurate  findings 
depend  upon  accurate  drill  logs;  and  that  such  records  are  essen- 
tial to  intelligent  geologic  work  as  well  as  the  economic  develop- 
ment of  oil  properties. 

METHOD  OF  ESTIMATING  WELL  DEPTHS  FROM  DRILLING  LINES 

In  many  cases  it  is  important  to  know  the  approximate  depth 
of  a  well.  Where  the  drillers  and  operators  refuse  to  give  out 
information,  the  following  method  may  be  employed: 

The  drilling  line  is  wound  on  the  bull-wheel  shaft  in  successive 
layers  or  coils,  each  consisting  of  several  turns  or  wraps  around 
the  shaft. 

The  tables  give  directly  the  length  of  line  in  each  coil. 

USE  OF  TABLES 

Example. — The  length  of  2J^-in.  Manilla  rope  on  a  bull-wheel 
shaft  having  1  coil  of  8  wraps  is  35.04  feet. 

The  total  length  of  line  on  a  bull  wheel  shaft  is  the  sum  of  the 
lengths  of  line  in  each  of  the  several  coils. 

Example. — The  length  of  %-in.  wire  line  on  a  bull- wheel  shaft 
having  3  coils  of  9  wraps  each,  and  a  fourth  coil  of  5  wraps  is  the 
sum  of 

The  length  of  line  in  the  1st    coil  of  9  wraps,  or  35.91  ft. 

The  length  of  line  in  the  2nd  coil  of  9  wraps,  or  39.42ft. 

The  length  of  line  in  the  3rd  coil  of  9  wraps,  or  42 . 93  ft. 

The  length  of  line  in  the  4th  coil  of  5  wraps,  or  25 . 80  ft. 


Which  is  a  total  length  of  144.06  ft. 

If  the  length  of  line  in  a  coil  of  more  than  10  wraps  is  required 
it  may  be  computed  by  multiplying  the  length  of  one  wrap,  as 
given  in  the  first  line  of  each  table,  by  the  number  of  wraps. 

Example. — The  length  of  %-inch  wire  line  on  a  bull  shaft  hav- 
ing 2  coils  of  20  wraps  each  is  the  sum  of 

The  length  of  line  in  the  1st   coil  of  20  wraps,  or  4.02  X  20  or     80.40  ft. 
The  length  of  line  in  the  2nd  coil  of  20  wraps,  or  4.48  X  20  or    89.60  ft. 

Which  is  a  total  length  of  170 . 00  ft. 


FACTORS  IN  OIL-WELL  DRILLING 


131 


TABLE  XIII 
TABLES  FOR  APPROXIMATE  WELL  DEPTHS 

Computed  for  Bull  Wheel  Shafts  14>^"  Dia. — Approx.  3'-9"  Circum. 
2}4-inch  Manilla  Rope 


Coils 

Wraps 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

1 

4.38 

5.56 

6.74 

7.92 

9.10 

10.28 

11.46 

12.64 

13.82 

15.00 

1 

.    2 

8.76 

11.12 

13.48 

15.84 

18.20 

20.56 

22.92 

25.28 

27.64 

30.00 

2 

3 

13.14 

16.68 

20.22 

23.76 

27.30 

30.84 

34.38 

37.92 

41.46 

45.00 

3 

4 

17.52 

22.24 

26.96 

31.68 

36.40 

41.12 

45.84 

50.56 

55.28 

60.00 

4 

5 

21.90 

27.80 

33.70 

39.60 

45.50 

51.40 

57.30 

63.20 

69.10 

75.00 

5 

6 

26.28 

33.36 

40.44 

47.52 

54.60 

61.68 

68.76 

75.84 

82.92 

90.00 

6 

7 

30.66 

38.92 

47.18 

55.44 

63.70 

71.96 

80.22 

88.48 

96.74 

105.00 

7 

8 

35.04 

44.48 

53.92 

63.36 

72.80 

82.24 

91.68 

101.12 

110.56 

120.00 

8 

9 

39.42 

50.04 

60.66 

71.28 

81.90 

92.52 

103.14 

113.76 

124.38 

135.00 

9 

10 

43.80 

55.60 

67.40 

79.20 

91.00 

102.80 

114.60 

126.40 

138.20 

150.00 

10 

24-inch  Wire  Line 


Coils 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

Wraps 

1 

3.99 

4,38 

4.77 

5.16 

5.55 

5.94 

6.33 

6.72 

7.11 

7.50 

1 

2 

7.98 

8.76 

9.54 

10.32 

11.10 

11.88 

12.66 

13.44 

14.22 

15.00 

2 

3 

11.97 

13.14 

14.31 

15.48 

16.65 

17.82 

18.99 

20.16 

21.33 

22.50 

3 

4 

15.96 

17.52 

19.08 

20.64 

22.20 

23.76 

25.32 

26.88 

28.44 

30.00 

4 

5 

19.95 

21.90 

23.85 

25.80 

27  .  75 

29.70 

31.65 

33.60 

35.55 

37.50 

5 

6 

23.94 

26.28 

28.62 

30.96 

33.30 

35.64 

37.98 

40.32 

42.66 

45.00 

6     . 

7 

27.93 

30.66 

33.39 

36.12 

38.85 

41.58 

44.31 

47.04 

49.77 

52.50 

7 

8 

31.92 

35.04 

38.16 

41.28 

44.40 

47.52 

50.64 

53.76 

56.68 

60.00 

8 

9 

35.91 

39.42 

42.93 

46  44 

49.95 

53.46 

56.97 

60.48 

63.99 

67.50 

9 

10 

39.90 

43.80 

47.70 

51.60 

55.50 

59.40 

63.30 

67.20 

71.10 

75.00 

10 

Wire  Line 


Coils 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

Wraps 

1 

4.02 

4.48 

4.94 

5.40 

5.86 

6.32 

6.78 

7.24 

7.70 

8.16 

1 

2 

8.04 

8.96 

9.88 

10.80 

11.72 

12.64 

13.56 

14.48 

15.40 

16.32 

2 

3 

12.06 

13.44 

14.82 

16.20 

17.58 

18.96 

20.34 

21.72 

23.10 

24.48 

3 

4 

16.08 

17.92 

19.76 

21.60 

23.44 

25.28 

27.12 

28.96 

30.80 

32.64 

4 

5 

20.10 

22.40 

24.70 

27.00 

29.30 

31.60 

33.90 

36.20 

38.50 

40.80 

5 

6 

24.12 

26.88 

29.64 

32.40 

35.16 

37.92 

40.68 

43.44 

46.20 

48.96 

6 

7 

28.14 

31.36 

34.58 

37  .  80 

41.02 

44.24 

47.46 

50.68 

53.90 

57.12 

7 

8 

32.16 

35.84 

39.52 

43.20 

46.88 

50.56 

54.24 

57.92 

61.60 

65.28 

8 

9 

38.18 

40.32 

44.46 

48.60 

52.74 

56.88 

61.02 

65.16 

69.30 

73.44 

9 

10 

40.20 

44.80 

49.40 

54.00 

58.60 

63.20 

67.80 

72.40 

77.00 

81.60 

10 

CHAPTER  VIII 
FACTORS  IN  OIL  PRODUCTION 

The  aim  of  every  operator  is  to  extract  the  maximum  of  oil  in 
the  minimum  of  time  and  at  a  minimum  cost.  To  accomplish 
this  he  must  map  out  a  definite  production  campaign.  Success- 
ful operating  depends  upon  the  ability  to  clearly  analyze  and 
comprehend  troubles  2000  ft.  or  more  underground.  Such  abil- 
ity often  saves  many  thousands  of  dollars  that  would  otherwise 
be  expended  in  futile  endeavors  to  remedy  some  of  the  troubles 
that  do  not  exist.  Geological  conditions  2000  ft.  underground 
may  seem  largely  a  subject  of  conjecture.  However,  an  excel- 
lent idea  of  such  conditions  can  be  obtained  by  a  careful  study  of : 

1.  Any  outcrops  of  the  oil  strata  and  the  beds  underlying 
and  overlying  them. 

2.  The  drill-logs  and  borings  from  individual  wells,  and 

3.  The  cross-sections  and  the  areral  maps  made  from  com- 
pilation of  a  number  of  drill-logs. 

From  the  first  sources  of  information,  some  clue  to  the  follow- 
ing may  be  obtained: 

(a)  The  hardness  of  the  formations. 
(6)  The  thickness  of  the  formations. 

(c)  The  dip  and  the  strike  of  the  oil  strata. 

(d)  The  size  of  the  sand  grain  of  the  oil  sands. 

(e)  Probable  water  sands. 

(/)  Evidences  of  faulting  and  minor  folds  that  may  affect 

the  oil  strata. 

From  the  second  sources  of  information  may  be  gained : 
(a)  Knowledge  of  the  actual  depths  to  the  oil  strata. 
(6)  The  number  of  oil  strata. 

(c)  The  true  thicknesses  of  the  oil  strata. 

(d)  The  true  thicknesses  of  the  overlying  beds. 

132 


FACTORS  IN  OIL  PRODUCTION  133 

(e)  The  relative  productivity  of  the  oil  sands. 

(/)  The  character  of  the  beds  overlying  the  oil  strata,  as 
regards  hardness,  and  whether  they  are  shales  or  sand- 
stones or  limestones. 

(g)  The  relative  position  and  number  of  water  sands. 

(h)  The  quality  of  the  water,  whether  alkaline,  saline, 

or  sulphurous. 
The  third  source  of  information  gives : 

(a)  The  areal  limits  of  the  several  oil  sands. 

(6)  The  areal  limits  of  water  sands. 

(c)  The  changes  of  oil  sands  to  water  sands. 

(d)  The  changes  of  coarse  sands  to  shales. 

(e)  The  true  dips  of  the  oil  sands. 

(/)  The    thickening   and    thinning   of   the   oil  strata   in 

accordance  with  overlap,  or  of  unconformity. 
All  of  the  information  outlined  above  is  of  great  value  to 
the  oil  man  who  is  planning  a  production  campaign.     Indeed, 
the  time  will  come  when  no  operator  will  begin  work  on  a  large 
scale  without  taking  these  factors  into  consideration. 

Problems- of  the  Producer. — Three  important  problems  eon- 
front  the  producer: 

1.  How   best   to   obtain   the   largest   quantity   of    petroleum 
from  his  own  property. 

2.  How  best  to  defend  his  property  from  drainage  by  neigh- 
boring properties. 

3.  How  best  to  drain  the  neighboring  properties. 

At  first  glance  problem  3  may  seem  to  call  for  illegal  actions. 
Petroleum  mining,  however,  differs  from  other  mining  in  that 
there  is  no  legal  liability  for  the  operator  who  depletes  a  neigh- 
boring property,  while  in  mineral  mining  no  operator  can  obtain 
ore  from  neighboring  property  without  being  responsible  for 
the  mineral  so  taken.  The  reason  for  this  difference  lies,  of 
course,  in  the  nature  of  the  substance  mined. 

Petroleum  is  a  fluid,  while  the  minerals  are  solids.  Petroleum, 
like  water,  seeks  its  level  and  also  tends  to  flow  through  any 
outlet  that  offers  easy  passage.  Such  being  the  case,  it  is  by 


134  PRACTICAL  OIL  GEOLOGY 

no  means  a  simple  matter  to  determine  the  quantity  of  fluid  that 
one  property  may  drain  from  specified  neighboring  properties. 
While  it  is  possible  to  tell  approximately  how  much  oil  has  been 
obtained  from  other  properties,  it  is  not  possible  to  tell  how 
much  oil  has  been  drained  from  a  certain  acreage,  specifying 
the  amounts  taken  from  each  neighboring  property.  Because 
of  this  uncertainty  no  legal  action  can  be  taken  against  an  opera- 
tor, even  though  his  oil  wells  may  have  completely  exhausted 
neighboring  territory. 

Amount  of  Oil. — Every  operator  desires  some  idea  of  the 
amount  of  oil  he  may  reasonably  expect  from  a  property.  Esti- 
mates of  this  kind  are  only  approximate,  but  are  useful  guides  to 
conservative  men.  The  following  factors  are  necessary  in  com- 
puting the  quantity  of  oil  that  a  property  may  reasonably 
produce: 

1.  The  number  of  oil  strata. 

2.  The  thickness  of  the  various  strata. 

3.  The  porosity  of  the  oil-bearing  formation. 

4.  The  areas  covered  by  each  stratum. 

The  first  two  factors  may  readily  be  secured  from  the  drill- 
logs.  The  third  factor  can  only  be  secured  roughly  by  a  study 
of  the  drill  cuttings  and  the  outcrops  of  the  oil  sands.  The 
fourth  factor  requires  careful  study  of  cross-sections  and  contour 
maps  made  from  a  compilation  of  many  drill-logs. 

Assume  a  property  (F)  covering  a  section  of  land — 640  acres 
in  proven  territory.  (See  Fig.  82a.)  Three  productive  sands 
are  known.  Drill  holes  Nos.  1  and  5  show  three  distinct  oil 
sands,  A,  B,  and  C,  with  thicknesses  averaging,  respectively, 
30,  25,  and  50  ft.,  but  with  a  gradual  thinning  eastward.  Drill- 
holes Nos.  6,  7,  and  8  show  two  sands,  B  and  C,  having  an  aver- 
age thickness  of  25  and  40  ft.,  respectively.  Drill  holes  Nos. 
9,  10,  and  11  show  but  one  oil  sand,  30  ft.  thick.  It  is  clear  that 
the  three  strata,  A,  B,  and  C,  do  not  all  underlie  the  entire  640 
acres  under  consideration. 

The  average  of  each  sand  that  underlies  the  property  is  a 
matter  of  approximation.  Drill  holes  Nos.  1  and  5  show  the 


FACTORS  IN  OIL  PRODUCTION 


135 


three  sands  (see  Fig.  826)  A,  B,  and  C,  while  Nos.  6,  7,  and  8 
show  B  and  C.  Evidently  the  top  sand  A  has  given  out  some- 
where between  Nos.  1  and  6  and  Nos.  5  and  8.  This  being  the 
case,  it  is  safe  to  assume  the  limits  of  the  A  sand  at  Nos.  1  and  5. 
This  gives  200  acres  covered  by  A,  and  possessing  an  average 
thickness  of  30  ft.  Sand  B,  as  shown  by  the  drill  records,  ex- 
tends to  Nos.  6,  7,  and  8,  which  are  taken  as  its  limits.  The 
area  covered  by  B  is  400  acres,  with  an  average  thickness  of  25 
ft.  C  covers  the  entire  640  acres,  with  an  average  of  40  ft.  All 


\ 

• 
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u 

V 

r 

\c 

• 

.  \ 

J 

• 

i  v 

t6 

+  9 

i 

i    R 

v  • 

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*3  i 

Direction 

ofDip*1^ 

^       » 

* 

1 

I 

I 

m 

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*     . 

•              U 

• 

•    ^^ 
"    j 

r 

• 
W 

V         ' 

X     / 

z 

{ 

1 

IB 

j 

IA 

1 

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FIG.  82a. — Shows  plan  of  wells  and      FIG.  826. — Shows  cross-section  in  di- 
limits  of  sands,  A,  B,  and  C.  rection  of  dip. 

told,  there  are  200  acres  30  ft.  thick,  400  acres  25  ft.  thick,  and 
640  acres  40  ft.  thick;  or  200  X  30  =  6000  acre-ft.,  +  400  X  25 
=  10,000  acre-ft.,  +  640  X  40  =  25,600  acre-ft.;  a  total  of 
41,600  acre-ft. 

This  total  is  a  fair  estimate  of  the  aggregate  amount  of  oil 
sand  underlying  the  section.  The  porosity  of  oil  formations 
differs  so  materially  that  no  constant  should  be  assumed  except 
as  an  approximation.  Sands  show  from  5  to  20  per  cent,  poros- 
ity, and  some  dolomitic  limestones  as  high  as  35  per  cent.  Gen- 


136 


PRACTICAL  OIL  GEOLOGY 


800 
280 
260 
240 


180 


160 


140 


120 


100 


60 


40 


20 


( ) 


10 


15 


20 


35 


40 


45 


50 


25    3 
Months 

FIG.  83c. — Typical  production  curves  of  wells  in  the  Mid-continent  oil 
field.  Curves  represent  production  under  leases  as  follows:  1,  Muskogee 
district,  Oklahoma;  2,  Bartlesville  district,  Oklahoma;  3  and  4,  Glenn  pool, 
Oklahoma.  (After  U.  S.  Bureau  of  Mines.) 


FACTORS  IN  OIL  PRODUCTION  137 

erally,  however,  10  per  cent,  is  accepted  as  the  basis  for  esti- 
mates. The  U.  S.  Geological  Survey  accepts  10  per  cent,  in 
most  of  its  computations.  One  gallon  per  cu.  ft.  is  often 
taken,  or  nearly  1000  barrels  per  acre-ft. 

For  the  purpose  of  illustration,  the  latter  estimate  will  be 
assumed.  Thus,  41,600  X  1000  =  41,600,000  bbl.  of  oil  under- 
lying the  property.  It  is  safe  to  assume  that  only  50  per  cent, 
of  this  oil  is  recoverable. 

Production  Curves. — Another  method  based  upon  the  history 
of  adjoining  leases  gives  an  excellent  means  of  arriving  at  esti- 
mates of  what  wells  under  similar  conditions  should  do. 

The  numbers  at  the  left  of  Fig.  83c,  page  110,  show  the  number 
of  barrels  of  oil  the  wells  produced. 

The  figures  at  the  bottom  give  the  number  of  months  the  wells 
have  produced. 

The  jagged  lines  show  the  production  from  month  to  month. 
As  one  can  see,  the  production  drops  steadily  the  longer  the  wells 
are  operated. 

Daily  decline  lines  or  curves  may  be  computed  in  the  same 
way. 

As  the  lines  show,  the  initial  or  flush  productions  are  large 
but  rapidly  decrease  with  age.  No.  1  shows  a  high  production 
of  312  bbls.  at  the  end  of  first  month;  120  bbls.  at  the  end  of  the 
second  month;  248  bbls.  the  end  of  the  third  month;  92  bbls.  the 
end  of  the  fourth  month;  75  at  the  end  of  the  fifth  month,  etc. 
At  the  end  of  the  first  year  the  production  was  20  bbls;  the  end 
of  the  second  year  was  8  bbls. ;  and  at  the  end  of  the  fourth  year 
less  than  4  bbls. 

Such  a  method  gives  an  excellent  working  basis  for  computing 
expected  performance  of  nearby  properties. 

Losses. — Experiments  made  by  the  U.  S.  Geological  Survey 
(see  Bull.  475,  pp.  2  to  16)  show  that  with  Pennsylvania  and 
Illinois  petroleum  of  0.810  (43°  B.)  and  0.8375  (37°  B.)  specific 
gravity,  respectively,  fuller's  earth  will  absorb,  on  an  average, 
40  per  cent,  of  oil  that  cannot  be  recovered.  Experiments  have 
not  yet  been  made  to  ascertain  the  percentage  of  oil  retained  by 


138  PRACTICAL  OIL  GEOLOGY 

the  coarser-grained  oil  sands.  With  heavier  oil  of  increased 
viscosity  it  is  safe  to  assume  fully  as  large  a  loss  as  that  given 
with  fuller's  earth. 

Added  to  this  loss,  by  retention  due  to  adhesion  and  friction, 
is  the  incomplete  drainage  of  the  oil  sands  by  drilled  wells.  This 
last  factor  is  most  important.  There  comes  a  time  when  the 
oil  accumulates  in  the  well  so  slowly  that  there  is  little  economy 
in  pumping  it.  When  this  point  is  reached,  the  oil  property  is 
abandoned  as  of  too  little  value  to  pay  for  operating.  It  is 
economically  exhausted,  though  not  actually  so.  The  above 
losses  do  not  take  into  account  possible  flooding,  cave-ins,  or 
drainage  by  neighboring  properties.  When  all  these  factors 
are  considered,  50  per  cent,  seems  too  high  an  estimate.  How- 
ever, for  general  purposes  this  figure  is  acceptable.  There  are, 
then,  20,800,000  bbls.  of  oil  that  may  be  extracted  from  the 
property  under  consideration.  Call  this  figure  20,000,000  bbls. 
for  safety. 

Acreage  per  Well. — The  next  step  is  to  find  the  number  of 
wells  that  will  be  needed  to  obtain  this  oil.  Here  again  as- 
sumptions are  necessary.  The  area  drained  by  one  well  is 
variously  estimated  at  from  1  to  10  acres.  Little  definite 
information  is  available  as  to  the  acreage  per  well  that  gives 
the  best  results.  Four  to  five  acres  is  accepted  as  good  practice, 
but  whether  or  not  this  is  the  best  figure,  cannot  be  said  at 
present.  Some  operators  space  their  wells  100  to  800  ft.  apart, 
varying  the  distance  inversely  with  the  richness  of  the  " sands." 
Taking  5  acres  as  a  basis,  128  wells  per  section  (640  acres) 
would  be  required. 

In  some  estimates,  1  well  to  8  acres  is  considered  ample, 
or  80  wells  per  section.  No  rule  can,  however,  be  set,  as  the 
porosity  of  the  sand,  the  gas  pressure,  and  the  dips  vary  greatly  in 
different  fields. 

The  more  porous  the  sand,  the  stronger  the  gas  pressure  and 
the  greater  the  dip,  the  less  number  of  wells  required  to  drain  a 
property.  With  compact  sandstones,  low  dips,  and  weak  gas 
pressures,  more  wells  will  be  required.  The  writer  has  personally 


FACTORS  IN  OIL  PRODUCTION  139 

known  wells  500  ft.  and  600  ft.  apart  to  draw  oil,  one  from  the 
other,  but  the  sands  were  porous,  the  dip  18  ft.  to  the  100  ft.,  and 
the  gas  pressure  large.  On  a  structure  with  steep  dips  it  is  bet- 
ter to  locate  wells  closer  together  across  the  dip  than  down  the 
dip. 

Dry  Spots. — In  some  producing  areas,  it  is  not  uncommon  to 
find  non-productive  spots  in  the  midst  of  producing  properties. 
Such  spots  may  be  due 

1.  To  local  hardening,  or  cementation  of  the  sand  grains. 

2.  To  island  conditions,  left  as  a  result  of  unconformities. 

3.  To  lensing  of  the  oil  sands. 

4.  To  deep  seated  faulting. 

None  of  these  conditions  can  be  foreseen,  unless  there  are  a 
number  of  drill  logs  available,  or  the  history  of  a  field  points  to 
such  conditions. 

Operating  Problems. — The  problem  of  securing  the  greatest 
quantity  of  petroleum  from  a  property  is  intimately  associated 
with  the  problems  of  securing: 

1.  The  greatest  quantities  of  petroleum  from  the  neighboring 
properties,. -and 

2.  Protecting  property  from  drainage  by  others. 

Many  other  minor  problems,  such  as  water  troubles  (sometimes 
a  serious  menace  to  a  whole  oil  field),  cave-ins,  collapsed  casing, 
leaks  due  to  filling  of  dry  sands,  and  others,  will  be  treated  after 
considering  the  main  problems  of  well-location.  Consider  the 
problems  involved  in  securing  oil  from  neighboring  properties. 
Fig.  82a  shows  a  number  of  properties  surrounding  F;  there 
are  U,  7,  and  W  above  Y  on  the  dip;  there  are  T  and  X  on  the 
sides  of  Y ;  and  there  are  S,  R  and  Z  below  F  on  the  dip.  The  three 
sets  of  properties  require  different  treatment  for  offense  and  de- 
fense. In  a  new  field  where  the  gas  pressure  is  strong,  the  oil 
generally  tends  to  migrate  upward  from  points  on  the  lower  dip. 
However,  in  olders  fields,  the  tendency  of  the  oil  is  to  move 
downward  under  the  effect  of  gravity.  Friction,  in  some  cases, 
especially  where  the  dip  is  slight,  will  keep  oil  from  traveling 
downward. 


140 


PRACTICAL  OIL  GEOLOGY 


o3 


Offensive  Tactics. — The  usual  practice  of  offense  consists  of 
the  following  elements. 

1.  To  sink  wells  before  neighboring  companies  drill. 

2.  To  drill  holes  of  larger  diameter  than  those  of  the  neighbor- 
ing companies. 

3.  To  drain  sands  unknown  to  the  neighbors. 

4.  To  place  the  wells  so  as  to  be  sure  of  draining  as  much 
territory  as  possible. 

5.  To  speed  the  wells  as  fast  as  possible. 

It  is  almost  a  self-evident  fact  that  the  first  well  in  a  field 
has  an  advantage  over  all  other  wells,  other  things  being  equal. 

If  one  company  completes  a  well 
a  month  ahead  of  another,  it  is 
but  natural  that  the  first  well 
will  take  the  cream  of  the  pro- 
duction. But  more  than  this  is 
likely  to  happen.  The  first  well 
may  form  channels  which  will 
effectually  drain  a  territory,  and 
a  neighboring  well  or  wells  will 
suffer  in  consequence.  (See  Fig. 
84.)  This  was  supposedly  the 
reason  for  the  barrenness  of  the 
wells  contiguous  to  the  Lake 
View  gusher.  In  this  case  the 
theory  seems  correct,  as  recent 
reports  indicate  that  wells  nearby  have  greatly  increased  in  pro- 
duction since  the  great  gusher  ceased  flowing. 

Casing  Sizes  in  Wells. — Wells  that  have  8-in.  casing  certainly 
have  an  advantage  over  wells  that  have  diameters  of  4  and  6  in. 
The  larger  casing  will  stand  more  rows  of  perforations,  which 
means  a  greater  surface  exposed  to  the  oil.  Also,  the  interior 
of  the  casing  will  give  a  larger  collecting  basin  for  the  oil.  In  this 
latter  case  the  ratios  of  the  larger  casing  to  the  small  will  be  82; 
62;  42  or  64  :36  : 16.  Thus  the  8-in.  hole  will  give  four  times  the 
collecting  basin  of  the  4-in.  hole  and  nearly  twice  the  basin  of  the 


o  4 


FIG.   84. — Illustrates  how  well  No.  1 
drains  oil  from   Nos.  2,  3,  4,  and  5. 


FACTORS  IN  OIL  PRODUCTION  141 

6-in.  hole.  However,  the  relative  exterior  surface  exposed  to  the 
oil  sands  will  not  bear  the  same  ratio.  Perforations  vary  greatly 
in  size,  from  %  by  %  in.,  %  by  2  in.,  J^  by  1  in.,  to  %  by  Sin.  and 
larger,  depending  upon  the  different  companies.  Generally  these 
perforations  are  in  longitudinal  rows  and  evenly  spaced,  one  every 
foot  or  6  in.  A  4-in.  casing  is  given  three  rows  of  perfora- 
tions, a  6-in.  casing  four  rows,  while  an  8-in.  casing  carries  five 
to  six  rows  of  machine-made  slots.  Where  a  rolling  knife  or 
casing  "splitter"  is  used,  there  is  little  regularity  of  rows.  If  the 
size  of  the  perforations  is  the  same  and  the  spacing  equal,  the 
ratios  are  6  to  4  to  3.  The  8-in.  casing  exposes  twice  the  surface 
of  the  4-in.  and  one  and  one-half  times  the  surface  of  the  6-in. 
The  advantage  then  is  certainly  with  the  larger  casing. 

It  must  not  be  supposed  that  the  increase  in  the  amount  of  oil 
obtained  is  in  proportion  to  the  ratio  of  collecting  area,  or  of  per- 
forated area.  However,  where  the  sands  are  fully  saturated,  the 
advantage  is  with  the  larger  casing. 

The  sizes  of  holes  given  above  apply  more  particularly  to  pump- 
ing wells.  Where  flowing  wells  occur,  smaller  holes,  such  as  3-in., 
have  of  necessity  been  used,  in  deep  territory,  3000  ft.  and  over. 
The  gas  pressures  in  flowing  wells  are  large  and  when  confined  to 
small  casings  have  greater  pressures  than  in  larger  holes.  In 
small  holes  one  can  obtain  large  productions,  3000  to  8000  bbl. 
with  less  danger  of  injuring  the  wells  than  by  letting  them  flow 
freely  through  larger  casings. 

Oil  Well  Screens. — Oil  well  "screens"  or  strainers  are  often 
used  instead  of  ordinary  perforated  casing.  Screens  or  strainers 
are  especially  adaptable  to  soft,  running  sands.  There  are  a 
number  of  types  of  screens  or  strainers  upon  the  market : 

(1)  The  plain  perforated  screen;  (2)  the  button  type;  and  (3) 
the  wire  wrapped  type. 

All  three  types  of  screen  have  been  used  but  the  most  successful 
and  popular  strainer  is  the  wire  wrapped  type. 

The  perforated  screen  consists  of  casing  with  horizontal  or 
vertical  slits  cut  into  the  pipe. 

The  button  type  is  made  by  drilling  circular  holes  in  the  casing 


142 


PRACTICAL  OIL  GEOLOGY 


and  then  placing  copper  buttons,  with  varying  sizes  of  slits  in 
them,  into  these  holes. 

The  wire  wrapped  screen  is  made  by  perforating  casing  as  in 
the  button  type  and  then  wrapping  the  casing  spirally  with  wire. 
The  wires  are  so  spaced  that  spaces  from  0.004  in.  to  J£  in.  can 
be  used.  The  wire  is  on  a  bevel  on  the  inside.  This  feature  of 
the  V-shaped  wire  is  to  prevent  any  particles  of  sand  that  work 
through  the  outer  face  of  the  screen  from  wedging  tight  around 
the  perforations.  (See  Fig.  85.)  The  per  cent,  of  the  open  space 
exposed  to  the  oil  sand  is  from  2  to  3  times  that  of  the  ordinary, 
perforated  casing. 


Fia.  85.— Oil  well  screen. 

Unknown  Oil  Sands. — It  may  seem  a  rare  occurrence  for  a 
company  to  tap  oil  sands  unknown  to  its  neighbors.  This,  how- 
ever, is  not  unusual,  especially  where  companies  have  failed  to 
prospect  for  deeper  sands.  It  is  then  discovered  too  late  that 
more  venturesome  neighbors  have  tapped  the  deeper  sands  and 
gained  a  good  production  at  the  first  concern's  expense,  having 
drained  much  of  the  property  of  the  careless  operator. 

Competitive  Adjacent  Properties. — Again,  companies  often 
wilfully  hold  back  information  regarding  their  prospect  holes, 
and  give  misleading  information  as  to  lower  sands.  However,  no 
company  is  under  moral  obligation  to  give  information  to  com- 
petitors, unless  prospect  holes  show  water  sands  that  may 


FACTORS  IN  OIS  PRODUCTION 


143 


endanger  the  neighboring  properties  as  well  as  their  own.  There 
are,  however,  cases  that  have  been  reported  where  concerns  have 
deliberately  flooded  a  field,  or  portions  of  it,  to  drive  the  oil 
from  neighboring  properties  to  their  own,  or  where  by  destroying 
neighboring  lands  these  companies  have  benefited  by  a  loss  of 
competition. 

The  question  of  placing  wells  so  as  to  drain  the  largest  possible 
territory  involves  some  interesting  problems.  One  case  in  par- 
ticular seems  worthy  of  study.  In  one  of  the  Oklahoma  fields, 
an  independent  operator  held  a  very  good  oil  property  which  he 
desired  to  develop.  Upon  investigation  it  was  found  that  the  only 


-375- — >•<    350 


R 


FIGJ  86. — Offensive  system  for  property  R. 

pipe  line  in  this  field  was  owned  by  a  company  which  owned 
held  leases  on  all  the  neighboring  land.  The  independent  operator 
could  not  afford  to  build  a  pipe  line  of  his  own,  and  the  cor- 
poration held  him  up  for  transportation.  Obviously,  the  only 
thing  he  could  do  was  to  keep  his  land.  Unfortunately  for  him, 
the  opposing  leases  were  lower  on  the  dip,  and  as  the  opponent  was 
developing  its  properties,  the  independent  man  was  forced  to 
give  up  hope  of  getting  any  oil  from  his  property.  The  stronger 
concern  would  not  buy  his  land,  so  that  in  time  the  property  was 
exhausted  without  the  independent  obtaining  anything  for  his 
share.  While  this  is  an  extreme  case  of  drainage,  the  same 
principle  applies  to  nearly  all  oil  fields  where  one  company  is 
favorably  situated  to  drain  oil  from  others. 


144  PRACTICAL  OIL  GEOLOGY 

The  principal  elements  considered  in  placing  a  well  to  best 
drain  a  neighboring  property  are:  (a)  The  spacing  of  wells,  and 
(6)  the  degree  of  dip  of  the  oil  sand. 

Where  an  opposing  company  has  already  sunk  its  wells,  there 
are  certain  lines  of  procedure  -that  may  be  successful.  If  its 
wells  are  far  apart  it  will  be  of  advantage  to  place  wells  at  inter- 
mediate points  as  well  as  opposite  the  other  wells.  (See  Fig.  86.) 
The  wells  on  Y  are  spaced  700  and  800  ft.  apart.  Wells  on  R 
may  then  be  spaced  350  and  400  ft.  apart.  This  will  force  the 
Y  operators  to  drill  wells  between  the  others.  This  being  the 
case,  the  R  operators  have  the  advantage  of  being  down  the  dip, 
and  by  placing  back  wells  at  points  intermediate  between  the 
first  row  of  wells  (see  Fig.  86),  effectually  cover  all  the  line  pre- 
sented by  F,  and  having  more  wells,  should  draw  the  oil  more 
rapidly  than  the  single  line  of  wells.  This  condition  is  by  no 
means  a  theoretical  one,  but  exists  in  a  number  of  places,  though 
often  quite  unnoticed. 

Speeding  Wells. — Speeding  the  wells  as  fast  as  possible  is 
one  of  the  means  of  obtaining  more  oil  than  neighboring  com- 
panies. But  there  is  a  decided  limit  to  such  speeding.  There 
must  first  be  oil  enough  to  pay  for  pumping  wells  rapidly.  In 
some  places  the  oil  seeps  into  the  well  basin  slowly  and  is  pumped 
off  in  a  few  hours.  Rapid  pumping  is  here  needless  and  expen- 
sive. In  other  places  there  is  plenty  of  oil,  but  rapid  pumping 
produces  unfavorable  conditions,  such  as  drawing  large  quantities 
of  sand  into  the  well,  and  consequent  trouble  with  the  well. 

Then,  too,  there  is  a  limit  to  the  speed  that  the  pump  rods 
(sucker  rods)  will  stand  without  breaking,  and  to  the  effect  of 
the  speed  upon  the  pumping  jack  or  upon  the  derrick  and  the 
machinery. 

In  some  cases  where  the  yield  of  the  wells  could  be  increased, 
the  effect  upon  the  machinery  might  result  in  serious  break- 
downs and  accidents.  A  rational  method  of  offense  would 
combine  all  the  elements  outlined  above. 

DEFENSIVE  TACTICS. — The  problem  of  defense  is  next  in  order. 
The  elements  to  be  considered  are  much,  the  same  as  those  of 


FACTORS  IN  OIL  PRODUCTION 


145 


offense.  If  a  neighboring  property  is  being  drilled,  wells  must 
be  drilled  to  offset  the  neighboring  wells.  If  a  neighbor  finishes 
his  wells  with  casing  of  large  diameter  he  must  be  met  with  the 
same  size  or  larger.  If  the  neighbor  penetrates  sands  not  before 
known  to  exist,  then  the  wells  must  be  deepened  to  offset  the 
wells  of  the  neighbor. 

If  there  are  two  properties,  W  and  X  (see  Fig.  87),  with  an 
anticline  running  through  them,  and  plunging  southwest  as 
shown  by  the  arrow  and  the  underground  contour  lines,  locating 
is  by  no  means  as  simple  as  most  operators  would  think.  If  a 
well  on  W  is  placed  on  the  axis  of  the  anticline,  the  X  operators 
without  studying  the  geological  side  of  the  question  would  ordi- 


North 


FIG.  87. — Offensive  method  employed  by  X  to  drain  W. 

narily  place  an  offsetting  well  at  location  No.  1.  The  location 
at  No.  1  does  not,  however,  properly  offset  the  W  well  which  is 
drawing  oil  from  both  flanks  of  the  anticline  and  along  the  axis. 
To  offset  the  W  well  place  a  well  on  X  at  No.  2.  By  placing  a 
well  in  this  way  No.  2  pulls  oil  from  both  sides  of  the  anticline 
and  being  lower  on  the  plunge  of  the  anticline  will  draw  oil  from 
the  W  well,  thus  effectively  protecting  the  X  property. 

Even  if  the  W  well  has  not  been  drilled  it  is  best  to  locate  a 
well  on  X  at  No.  2,  as  a  well  at  that  location  draws  from  both 
sides  of  the  anticline  and  from  below.  To  offset  the  X  well  the 
best  possible  location  on  W  would  be  on  the  axis  of  the  anticline. 
Even  then  the  W  well  is  at  a  disadvantage  as  it  is  up  the  plunge 

from  No.  2  and  the  oil  will  naturally  gravitate  to  No.  2, 

10 


146 


PRACTICAL  OIL  GEOLOGY 


Often  (indeed,  in  most  cases)  a  property  may  embody  the 
principles  of  offense  against  properties  higher  on  the  dip  and 
defense  against  properties  lower  on  the  dip.  Thus,  in  Fig.  82a, 
the  properties  U,  V,  W,  T,  and  X  are  on  the  defensive  against 
F;  and  Y  is  on  the  defensive  against  properties  S,  R,  and  Z\ 
also,  in  Fig.  86,  F  is  on  the  defensive  against  R.  In  this  latter 
case,  when  wells  are  drilled  on  R  intermediate  between  those  on 
Y,  the  operators  on  F  should  have  met  this  attack  by  drill- 
ing wells  to  offset  those  on  R.  When  the  R  wells  are  pumped 
rapidly  the  F  wells  must  be  speeded  to  meet  the  increased  drain. 
Indeed,  the  F  property  is  at  a  decided  disadvantage  as  regards 
attacking  R.  Such  being  the  case,  all  that  can  be  done  is  to 
keep  up  as  vigorous  a  defense  as  is  possible  under  the  conditions. 
Different  systems  of  offense  and  defense  have  been  worked  out 
by  many  companies.  However,  the  problems  presented  here 
embody  the  essential  features  of  such  systems. 

Key  Wells.— 1An  interesting  application  of  geology  occurs  es- 
pecially in  the  Kern  River  field  of  California,  where  water 


Key 
Well 


FIG.  88a.- 


-Relation  of  key  well  to 
other  wells. 


FIG.    886. — -Cross-section    showing 
drainage  by  key  well. 


troubles  are  greatly  lessened  by  what  is  called  the  " key-well" 
system.  Fig.  88a  shows  six  wells  located  on  a  monocline.  The 
direction  of  dip  is  shown  by  the  arrow.  The  cross-section  (Fig. 
886)  shows  the  underground  structure. 


FACTORS  IN  OIL  PRODUCTION 


147 


Water  naturally  gravitates  toward  the  lower  well.  By  using 
an  air-compressor,  water  may  be  pumped  from  the  formation 
very  rapidly.  By  this  method  of  pumping,  one  well  can  with- 
draw most  of  the  water  from  the  oil  sand  and  lower  the  percentage 
of  water  in  the  petroleum  that  comes  from  the  other  wells.  In 
some  cases,  production  has  been  increased  from  100  to  500  per 
cent,  besides  lowering  the  water  content.  The  explanation  of 


FIG.  89. — Shows  oil  entering  dry  sand  below. 

this  increase  in  production  seems  to  be  in  the  fact  that  water  and 
oil  form  a  heavy  emulsion  that  does  not  pump  readily  and  also 
seems  to  clog  the  water  sands.  The  decrease  in  water  pressure 
also  keeps  the  water  from  driving  back  the  oil. 

The  method  shown  above  may  be  reversed.  In  Pennsylvania 
and  West  Virginia  water  is  introduced  into  the  lower  well  under 
great  pressure.  The  water  forces  the  oil  up  the  dip  to  the  oil 


148  PRACTICAL  OIL  GEOLOGY 

wells.  Wells  that  produce  from  y±  to  1  bbl.  have  gained  to  3 
and  4  bbls.  per  day.  Of  course,  such  a  procedure  is  not  sanc- 
tioned by  law,  but  it  has  been  employed,  nevertheless. 

Compressed  air  has  been  employed  to  recover  oil.  The  air  is 
forced  into  the  oil  zone  through  a  well,  or  wells,  and  the  oil  is 
forced  to  migrate  from  the  neighborhood  of  the  key  well  or  wells 
to  wells  that  are  pumping  oil.  Compressed  air,  used  in  such  a 
way  is  expensive,  but  favorable  results  have  been  achieved. 

Vacuum  pumps  are  used  in  many  places  to  secure  casing  head 
gas,  and  also  to  increase  oil  production.  The  suction  induced  by 
a  vacuum  is  sufficient  to  cause  a  flow  of  oil  to  the  wells  on  which 
the  vacuum  pumps  are  used,  and  to  completely  rob  other  neigh- 
boring wells  not  employing  a  vacuum. 

Dry  Oil  Sands  (See  Fig.  89). — Dry  oil  sands  may  cause  a 
loss  of  production  little  suspected  by  many  operators.  Where 
wells  are  drilled  deeper  than  the  productive  strata,  and  are  left 
open  or  free  of  casing  below  the  oil  sands,  petroleum  will  fill  the 
lower  dry  sands  and  escape.  This  condition  occurs  in  some 
wells  that  have  been  drilled  carelessly.  Wells  are  sometimes 
pumped  far  below  the  productive  strata.  Such  cases  are  due  to  a 
carelessness,  but  they  exist.  Where  there  are  holes  in  the  casing 
similar  leakage  may  result.  The  remedy,  of  course,  in  the  latter 
case  is  to  replace  the  casing;  in  the  former,  to  plug  the  hole  just 
below  the  oil  sand. 

Gas  will  escape  in  a  similar  manner.  Indeed  as  gas  can  travel 
where  oil  cannot,  gas  is  even  more  likely  to  escape  than  oil. 

Underground  Mapping. — In  many  fields,  after  careful  cross- 
sectioning  and  contouring,  it  has  been  shown  that  in  places  pro- 
ductive sands  have  been  passed  through  without  being  known. 
Later,  by  perforating  the  casing  in  the  old  wells  or  by  drilling 
new  ones,  production  has  been  obtained  from  the  neglected  sands. 

Practical  Application  of  Structure  Contour  Maps. — A  properly 
made  structure  contour  map  will  bring  out  points  that  may  save 
many  thousands  of  dollars  in  later  oil  fields  development.  Two 
illustrations  of  such  usage  are  enough  to  show  some  of  their 
advantages  to  the  oil  man. 


FACTORS  IN  OIL  PRODUCTION 


149 


Case  I. — Fig.  90  shows  a  gas  well  at  No.  1,  an  oil  well  at  No.  2 
and  a  dry  hole  at  No.  3.  The  question  now  is  where  should  the 
owner  of  "X"  locate  next  to  secure  a  well.  The  producing  well 
at  No.  2  is  practically  on  the  980  level;  No.  3  is  just  below  the 
980  contour.  By  following  it  around  it  is  seen  that  the  edge  of 
that  level  is  near  the  edge  of  "X."  Therefore  there  is  little 


FIG.  90. 


chance  for  property  "X"  to  obtain  wells  below  the  980  line,  and 
the  property  should  be  abandoned.  Likewise  No.  2  sets  the 
limit  of  drilling  on  the  "R"  property.  However,  a  dry  hole  at 
No.  4  on  the  West  is  shown  above  the  contour  line  of  970,  but 
production  is  not  limited  there.  This  is  notably  the  case  where 
the  normal  dip  is  broken  by  a  short  reversed  dip. 

Case  II. — A  property  is  for  sale.  What  will  be  the  possibilities 
for  oil  on  that  property?  From  the  map  it  is  plain  that  there  is  a 
chance  to  obtain  as  good  producing  wells  on  "  7"  as  on  "72,"  as 


150  PRACTICAL  OIL  GEOLOGY 

the  980  contour  line  includes  all  but  a  small  portion  of  "F" 
within  its  limits. 

Property  "Z"  will  be  of  little  value  though  there  is  a  possi- 
bility of  a  line  of  wells  along  the  north  boundary  on  the  same  level 
as  well  No,  2. 


CHAPTER    IX 
WATER,  THE  ENEMY  OF  THE  PETROLEUM  INDUSTRY 

There  are  two  main  sources  of  water  in  all  oil  fields:  water 
confined  in  the  oil-bearing  strata,  and  water  occurring  separate 
from  the  oil  zones.  In  the  first  case  water  is  sealed  in  the  oil  sand 
and  normally  underlies  the  oil  and  gas.  It  may  be  sulphur  or 
salt  water,  or,  in  fact,  may  contain  many  minerals.  Originally 
it  was  meteoric  or  rain  water  that  percolated  through,  the  earth 
until  it  reached  the  oil  strata,  where  it  was  sealed  or  held  in  by  the 
shaly  or  clayey  beds  that  generally  are  found  above  and  below 
the  oil  sands.  In  some  cases,  however,  this  water  is  the  sep, 
water  penetrating  the  shales  and  sands  before  folding  occurrecj. 
This  water  is  generally  under  rock  or  gas  pressure  and  as  it  is  al- 
ready in  the  oil  zone,  there  is  no  way  of  eliminating  it  as  a  source 
of  trouble.^  However,  where  there  are  two  or  more  oil  zones 
and  but  one  is  troubled  with  water,  there  may  be  great  economy 
in  shutting  off  one  of  the  troublesome  zones.  On  the  whole, 
water  confined  to  the  oil  zone  is  less  dangerous  to  the  industry 
than  water  occurring  in  other  than  the  oil  zone.  It  may  occur 
above,  below,  or  between  the  oil  strata.  (See  Fig.  94.)  Gen- 
erally under  head  and  occurring  in  large  quantity,  it  becomes  a 
source  of  great  danger  to  an  oil  field  when  it  enters  the  oil  zones; 
indeed,  it  is  the  water  most  to  be  feared  and  to  be  guarded 
against.  Obviously,  such  water  enters  the  oil  zones  only  as  the 
result  of  artificial  or  man-made  causes,  as  the  oil  zones  and  water- 
bearing strata  are  separate  and  distinct.  Earthquakes  or  volcanic 
upheavals  may  cause  faulting  and  shattering  of  the  formations 
to  such  an  extent  that  water  enters  the  oil  zones  from  the  separate 
water  sands,  but  for  all  practical  purposes  the  latter  causes  will 
not  be  considered. 

Two  sources  of  water  have  been  discussed.  For  the  sake  of 

151 


152 


PRACTICAL  OIL  GEOLOGY 


clearness  all  those  waters  originally  confined  in  the  oil  zone  will 
be  called  primary  waters,  and  all  waters  occurring  in  strata 
separately  from  the  oil  zones  and  entering  the  oil  zones  from 
artificial  causes  will  be  called  secondary  waters.  These  defini- 
tions refer  to  the  original  sources  of  the  water.  The  terms 
" bottom  water/'  "surface  water7'  and  "edge  water"  are  in 
constant  use  by  oil  men  to  express  the  occurrence  of  water  in 
oil  zones,  especially  in  newly  drilled  wells.  As  these  terms  do 
not  take  into  account  the  source  of  the  water,  whether  or  not 
it  was  originally  confined,  in  the  oil  zone  by  natural  processes 
or  whether  or  not  it  was  introduced  by  artificial  means,  the  more 
exact  classification  as  given  above  will  be  used. 

Primary-water  Troubles. — Primary  water,  as  before  mentioned, 
lies  at  the  bottom  of  the  oil  sand  under  pressure.     As  the  gas  and 


FIG.  91.— Water  in 
of  zone 


irtial  control 


Fia.  92. — Water  in  full  control  of 
B  and  entering  A. 


oil  are  withdrawn  from  the  sand  the  water  rises  to  replace  them. 
In  time,  nearly  all  the  oil  is  drawn  from  the  sand  and  only  the 
water  is  left.  The  field  must  then  be  abandoned.  This  is  the 
result  of  natural  exhaustion  and  is  expected  in  every  field. 
Where  there  is  but  one  oil  zone,  little  avoidable  danger  or  trouble 
occurs.  Assuming,  however,  two  strata  (Fig.  91),  as  commonly 
is  the  case,  water  will  rise  from  the  lower  stratum  B  into  the 


WATER,  THE  ENEMY  OF  THE  PETROLEUM  INDUSTRY  153 

upper  stratum  A,  unless  proper  precautions  are  taken  to  avoid 
this  condition.  Where  the  gas  pressure  is  strong  the  water  in 
B  will  be  driven  into  A.  Especially  is  this  true  when  zone  B 
becomes  all  or  nearly  all  water.  (See  Fig.  92.)  By  properly 
confining  the  water  to  the  bottom  zone,  water  troubles  would  be 
averted  for  a  time.  Again,  there  is  an  extreme  condition,  as 
shown  in  Fig.  70,  in  which  the  water  takes  possession  of  A 
and  then  enters  B.  This  condition  requires  that  the  water  in 
A  be  kept  from  B  by  methods  differing  from  those  used  in  the 
two  previous  cases.  These  methods  will  be  discussed  later. 


FIG.  93. — Water  in  partial  control    FIG.  94. — Water  between  A  and  B  en- 
of  A  and  beginning  to  flood  B.          teringboth  sands.     Water  above  A. 

Figs.  92  and  93  illustrate  the  extremes  of  water  flooding. 
These  conditions  are  theoretical,  but  are  closely  approximated 
under  actual  conditions.  Generally  all  the  oil  is  not  driven 
from  the  well,  but  some  remains  as  an  emulsion  of  finely  particled 
oil  and  water  which  is  very  difficult  to  treat.  However,  the 
two  cases  given  set  the  limits  within  which  all  primary-water 
troubles  of  the  flooding  type  may  be  placed. 

Secondary-water  Troubles. — Secondary  water  enters  the  oil 
zones  due  to  four  causes,  namely:  (1)  Accidents  to  the  casing, 
used  in  shutting  off  the  water  sand;  (2)  faulty  cementing  of  the 


154 


PRACTICAL  OIL  GEOLOGY 


water  sand;  (3)  cave-ins  due  to  the  withdrawal  of  a  large  quantity 
of  sand  from  the  oil  stratum,  thus  allowing  water  to  enter  the 
oil  zone  from  above;  and  (4)  where  no  effort  has  been  made  to 
shut  off  the  water  formation,  especially  in  prospect  holes. 

Accidents  to  the  casing  that  shuts  off  the  water  may  result 
from  the  dropping  of  sharp-pointed  tools,  bailers  or  sand  pumps 
into  the  hole,  or  from  falling  tubing.  The  casing  may  be  cor- 
roded by  the  action  of  the  minerals  in  the  water,  and  later 
collapse.  The  eroding  action  of  sand  in  a  flowing  well  is  often 

sufficient  to  cut  the  casing. 
Sometimes  the  casing  is  de- 
fective or  is  not  put  together 
properly.  Again,  the  sudden 
shifting  of  the  sands  in  the 
oil  zone  may  cause  the  casing 
to  pull  apart  or  break  at  the 
water  string. 

Accidents    to    the   cement 
may   result    from    the  same 
causes  as  enumerated  above, 
without,    however,    affecting 
the  casing    to    any    marked 
degree.     Again,    the    cement 
FIG.  95.— Water  from  top  stratum    may    be    improperly    mixed; 
entering  zones  A  and  5  due  to  caving    ^e  action  of  the  water  upon 
of  shale  above  A  resulting  from  with-    .  . 

drawal  of  sand.  it  may  destroy  its  efficiency; 

or  it  may  be  so  porous  that  it 

will  not  withstand  the  water  which  seeps  through  it,  and  in 
time  wears  large  channels.  Where  the  water  pressure  is  great, 
the  cement  may  be  ineffective. 

Cave-ins  around  the  casing  (Fig.  95)  are  of  common  occur- 
rence in  the  California  fields. 

Where  large  quantities  of  sand,  often  50,000  to  100,000  cu. 
ft.,  are  taken  from  the  oil  zones,  there  must  of  necessity  be  left 
some  form  of  cavity  underground.  This  cavity  leaves  the  roof 
above  it  unsupported.  Where  this  roof  consists  of  soft  shale  or 


WATER,  THE  ENEMY  OF  THE  PETROLEUM  INDUSTRY  155 

clay  cave-ins  are  inevitable.  If  the  distance  between  the  oil 
zone  and  the  water  stratum  is  slight,  such  a  cave-in  would  as- 
suredly admit  water  to  the  oil  sand.  As  this  question  is  one  of 
great  importance  a  fuller  discussion  will  be  given  farther  on. 

In  cases  where  no  effort  is  made  to  shut  off  water  the  con- 
sequences are  often  very  dangerous  to  the  life  of  the  field.  Then 
the  water  has  free  scope  and  soon  floods  the  near-by  portion  of 
the  field  and  later  may  spread  over  a  large  extent  of  good  terri- 
tory. " Wildcat"  drillers  are  especially  prone  to  neglect  the 
proper  precautions  to  shut  off  water.  When  the  speculator 
strikes  oil,  he  sells  his  property  and  leaves  the  purchaser  to 
shoulder  the  burden  of  responsibility.  Sometimes  these  prop- 
erties stand  idle  a  long  time  and  in  consequence  become  worthless. 

Determination  of  the  Source  of  Water. — It  is  no  easy  matter 
to  determine  the  source  of  the  water  or  the  cause  of  its  appear- 
ance in  a  well.  To  effectually  remedy  water  evils,  such  informa- 
tion is  essential. 

There  are  four  sources  of  evidence  from  which  one  may  draw 
the  needed  information,  as  follows:  (1)  Evidence  derived  from 
the  chemical  and  physical  properties  of  the  water  itself;  (2)  from 
the  neighboring  wells  as  shown  by  their  structural  relation  to 
the  well  in  question;  (3)  from  the  drill  logs,  and  (4)  from 
mechanical  tests  made  on  the  wells  by  using  drilling  tools,  plugs, 
bailers,  pumps,  testers,  etc. 

1.  CHEMICAL  AND  MECHANICAL  TESTS  OF  WATER. — The 
evidence  derived  from  the  study  of  the  chemical  and  physical 
properties  of  the  water  is  unreliable  except  in  a  few  special 
cases.  Both  the  primary  and  secondary  waters  may  have  the 
same  composition  due  to  their  similar  origin.  This,  however, 
is  not  always  the  case.  Sometimes  the  primary  water  may  be 
saline  and  the  secondary  sulphurous. 

In  this  case,  or  in  similar  cases,  no  difficulty  would  appear. 
There  are,  however,  oil  fields  in  which  the  secondary  water  is 
saline  and  sulphurous  in  different  parts  of  the  field,  but  in  the 
same  geological  horizon.  Obviously,  the  occurrence  of  salt  or 


156  PRACTICAL  OIL  GEOLOGY 

sulphur  water  in  the  oil  zone  would  mean  nothing  definite  in 
such  a  case. 

Where  careful  tests  have  been  made  of  all  the  waters  in  a 
field,  the  presence  of  a  new  or  unusual  water  in  the  oil  zone  would 
point  to  a  primary  water.  It  is,  however,  almost  impossible 
to  obtain  careful  analyses  of  all  the  waters  in  a  large  oil  field  and 
to  properly  correlate  them.  Waters  in  the  same  horizon  and 
but  a  quarter  of  a  mile  apart  often  differ  greatly  in  their  per- 
centage composition  of  salts.  Again,  the  waters  in  the  oil  zone 
may  show  marked  differences.  Such  being  true,  it  is  obvious 
that  little  reliance  can  be  placed  in  chemical  and  physical  tests 
unless  exhaustively  made  to  completely  cover  an  entire  field. 
Taken  in  conjunction  with  other  evidence,  however,  the  chemical 
and  physical  differences  of  waters  may  be  of  marked  value 
in  assisting  one  to  determine  the  source  of  the  water. 

2.  EVIDENCE  OF  NEIGHBORING  WELLS. — The  second  source  of 
evidence  gives  some  valuable  aids.  If  the  water  is  confined  to 
but  one  well  while  the  wells  on  all  sides  show  no  water,  then  the 
water  must  be  local  and  secondary.  In  such  a  case  look  for 
defective  casing,  defective  cement,  or  for  a  cave-in. 

If  the  neighboring  wells  higher  on  the  dip  of  the  anticline, 
monocline,  terrace  or  other  structural  feature  show  water,  while 
the  wells  below  do  not,  then  the  water  is  secondary  and  one  must 
look  for  causes  in  keeping  with  such  a  source. 

If  the  wells  below  show  water,  while  the  wells  above  do  not, 
then  the  evidence  points  to  primary  water.  This,  however,  is 
not  conclusive  unless  it  is  known  that  the  lower  wells  have  no 
casing  troubles  or  give  no  evidence  of  cave-ins. 

If  the  wells  both  above  and  below  show  water,  then  the  evi- 
dence points  to  either  primary  or  secondary  water,  unless  the 
percentage  of  water  in  the  wells  increases  lower  down  the  dip. 
In  such  a  case  the  evidence  points  to  primary  water.  This  is  not 
conclusive  by  any  means,  since  the  closer  one  approaches  to  a 
leaky  well  the  greater  is  the  percentage  of  water  in  the  oil.  This 
evidence,  however,  gives  one  a  clue  that  may  prove  of  great  value 
in  solving  water  problems. 


WATER,  THE  ENEMY  OF  THE  PETROLEUM  INDUSTRY  157 

INDICATORS. — Indicators  have  been  successfully  used  in  many 
cases  to  trace  the  source  of  certain  waters,  to  determine  the  rate 
of  travel  of  underground  waters  and  like  data.  As  applied  to 
oil,  their  use  is  quite  limited.  Such  tests  have  in  some  cases' 
proved  successful.  These  cases  were  where  the  wells  were  close 
together.  Where  two  or  more  wells  are  within  300  to  500  ft.  of 
one  another,  indicators  can  be  used  successfully,  but  it  is  by  no 
means  easy  to  test  wells  several  hundred  yards  apart  by  this 
means.  The  rate  of  travel  of  the  indicators  is  so  slow,  the  area 
over  which  the  indicator  will  spread  so  large,  and  the  final 
quantity  of  indicator  received  so  small,  that  it  is  out  of  the 
question  to  use  indicators  successfully  unless  a  well-defined 
channel  exists  between  two  wells. 

3.  EVIDENCE  OF  THE  DRILL  LOGS. — The  third  source  of 
evidence,  drill  logs,  is  extremely  useful.  The  drill  logs  show 
the  number  of  water  sands  and  their  relation  to  the  oil  sands. 
From  these  logs  one  can  also  determine  the  thickness,  the  hard- 
ness, and  the  quality  of  the  strata  lying  between  the  oil  zone  and 
the  water -formation.  The  character  of  the  oil  sand  itself  is  also 
shown  in  properly  kept  logs.  These  facts  are  all  important  in 
determining  the  probable  conditions  in  the  well. 

If  the  roof  or  capping  overlying  the  oil  sand  is  several  hundred 
feet  thick,  the  probability  of  a  cave-in  is  greatly  lessened.  If 
the  roof  consists  of  a  hard  sandstone,  or  of  shale  interlain  with 
sandstone  layers  several  feet  thick,  the  probability  of  cave-ins 
admitting  water  is  almost  nil.  Where  a  few  feet  of  shale  sepa- 
rate the  water  formation  from  the  oil  zone,  cave-ins  would 
undoubtedly  admit  water  to  the  oil  zone. 

If  the  oil  formation  consists  of  a  coarse  gravel,  cave-ins  would 
be  precluded.  The  fine  sand  is  drained  from  the  gravel  leaving 
minute  interstices  between  the  pebbles  but  no  large  cavity,  con- 
sequently an  unsupported  roof  is  not  very  likely. 

Cave-ins  result  in  the  presence  of  a  large  quantity  of  shale  in 
the  oil,  in  collapsed  casing,  and  often  in  a  diminished  production 
of  oil.  Underground  cavities  play  a  more  important  part  in  the 
life  of  an  oil  well  than  is  generally  accredited  to  them.  .Especially 


158  PRACTICAL  OIL  GEOLOGY 

is  this  true  in  the  California  and  the  Russian  oil  fields,  where  the 
oil  strata  consist  generally  of  unconsolidated  sands  that  are 
readily  pumped  from  the  well.  The  presence  of  dangerous 
cavities  would  seem  to  depend  in  these  fields  upon  the  dip  of  the 
oil  sands.  Surface  observations  show  that  saturated  oil  sand 
has  an  angle  of  repose  varying  between  10°  and  15°,  or  a  slope  of 
between  17  and  24  ft.  in  100  ft.  The  greater  the  degree  of  satura- 
tion the  less  the  angle  of  repose  within  the  limits  imposed.  Such 
being  the  case,  it  would  follow  that  where  the  dip  of  the  oil 
stratum  is  equal  to  or  greater  than  the  angle  of  repose  no  cavities 
would  exist  around  the  casing.  The  angle  of  repose  would  be 
modified  to  some  extent  by  the  gas  pressure  and  also  by  the  back 
pressure  exerted  by  the  column  of  oil  that  rises  in  the  casing. 
The  underground  angle  of  repose  should,  however,  approximate 
the  angles  determined  from  observations  taken  as  the  sand  stands 
in  the  sump  holes.  As  the  likelihood  of  cavities  depends  upon 
the  degree  of  dip  of  the  oil  strata  it  is  evident  that  where  the 
angle  of  repose  is  greater  than  the  degree  of  dip,  cavities  would 
occur  around  the  casing. 

4.  EVIDENCE  OP  MECHANICAL  TESTS. — Mechanical  tests  are 
any  tests  made  by  means  of  the  tools  and  other  implements  used 
in  drilling  operations.  These  tools  are  generally  bailers  or  sand 
pumps  (see  Fig.  73),  packers  and  plugs  (see  Figs.  97  and  98), 
casing  testers,  and  lifting  pumps.  Swedges  may  also  be  included. 
By  placing  a  casing  tester  below  the  water  string  it  is  a  simple 
matter  to  test  for  leaks  in  the  casing. 

The  casing  tester  is  a  temporary  plug  that  will  not  allow  water 
or  oil  to  pass  either  above  or  below  it,  as  it  fits  snugly  inside  the 
casing.  The  oil  and  water  above  the  tester  are  bailed  or  pumped 
out  of  the  hole  and  the  tester  is  then  left  in  the  well  a  few  hours. 
Later  the  hole  is  bailed  or  pumped  out  and  the  fluid  examined 
for  evidence  of  water.  The  same  method  applies  to  testing  the 
oil  zones  for  water.  The  casing  tester  is  placed  below  the  top 
oil  zone  and  this  zone  is  then  tested  either  by  bailing  or  pumping. 
If  it  is  desirable  to  test  the  lower  zone  a  different  procedure  is 
necessary.  A  tubing  packer  is  placed  low  enough  on  a  string 


WATER,  THE  ENEMY  OF  THE  PETROLEUM  INDUSTRY  159 


of  tubing  to  shut  off  the  upper  oil  zone  or  zones  and  the  lower 
zone  is  then  tested  by  means  of  a  lifting  pump  or  by  using  a 
bailer  of  a  smaller  diameter  than  the  tubing.  If  the  casing  has 
collapsed  and  will  not  admit  a  tester,  a  s wedge  (see  Fig.  99) — a 
spindle-shaped  and  somewhat  bulbous  tool — is  introduced  into 
the  hole  and  used  to  clear  a  passage  for  the  tubing. 

By  means  of  mechanical  tests  the  condi- 
tion of  the  casing  is  determined.  The  partic- 
ular oil  zone  or  zones  that  are  troubled  with 
water  are  shown  and  leaks  in  the  water  string 
are  located.  Further  than  this  the  mechani- 
cal tests  cannot  go.  If  water  is  present  in  the 
oil  zones,  it  may  be  primary  or  secondary.  If 


Swedge 


FIG.   96. — Dart  bot- 
tomed bailer. 


FIG.  97.— Wall 

packer. 


FIG.  98.— Rub- 
ber plug. 


FIG.  99.— 
Swedge. 


the  latter,  it  will  come  from  neighboring  wells  or  cave-ins  around 
the  casing;  if  the  former,  it  will  show  in  wells  lower  on  the  dip. 
Such  being  the  case,  it  is  clear  that  mechanical  tests  must  be  used 
in  conjunction  with  the  other  sources  of  evidence  to  obtain  the 
best  results.  No  single  source  of  information  furnishes  absolute 
proof. 


160  PRACTICAL  OIL  GEOLOGY 

Treatment  of  Primary  Water  Troubles. — Primary  water,  as 
already  stated,  ultimately  replaces  the  oil  in  every  field.  This 
being  true,  the  only  course  left  to  the  operator  is  to  prolong  the 
life  of  the  well  by  taking  every  proper  precaution  to  take  the  oil 
from  above  the  water  level.  If  the  primary  water  level  is  known, 
it  is  not  a  difficult  matter  to  place  a  wooden  plug  or  bottom  packer 
a  few  feet  above  the  water  level.  In  this  way  the  oil  is  taken 
from  the  well  without  taking  much  if  any  water.  Slow  pump- 
ing is  also  required  so  that  the  water  will  not  be  disturbed  and 
drawn  out  with  the  oil  as  an  emulsion  of  oil  and  water.  As 
the  water  rises  new  plugs  must  be  inserted  until  the  oil  is  exhausted. 

Where  there  are  two  or  more  oil  zones,  one  of  which  has  gone  to 
water,  it  is  best  to  isolate  the  faulty  zone.  This  may  be  done 
in  several  ways :  where  a  lower  zone  causes  the  trouble  a  bottom 
packer  placed  above  the  lower  zone  and  below  the  upper  zone  or 
zones  effectually  keeps  the  water  shut  off.  A  bottom  packer 
consists  of  a  rubber  plug  with  a  tapering  hole  in  it.  A  mandrel  is 
driven  into  this  hole  (see  Fig.  98)  causing  the  rubber  to  expand, 
thus  effectually  plugging  off  the  lower  stratum.  Where  the 
troublesome  zone  is  an  upper  zone,  other  methods  must  be  used. 
In  this  case,  the  stratum  will  be  treated  as  a  water  stratum  and 
shut  off  by  means  of  wall  packers,  seed-bags,  cementing  methods, 
or  by  setting  the  casing  in  a  clayey  or  shaly  bed.  These 
methods  require  the  pulling  of  the  casing  and  perhaps  a  redrilling 
of  the  well. 

Wall  packers  (see  Fig.  97)  consist  of  two  metal  cylinders 
between  which  is  placed  a  rubber  cylinder,  secured  to  the  metal 
cylinders  by  some  suitable  device.  By  putting  pressure  upon 
the  metal  cylinders  the  rubber  is  caused  to  expand.  This  device 
is  placed  on  the  outside  of  the  casing  and  by  means  of  a  spring, 
or  simply  by  means  of  the  pressure  of  the  casing,  the  rubber  is 
caused  to  expand,  closing  the  space  between  it  and  the  wall  of 
the  drill  hole  There  are  several  different  styles  of  wall  packers, 
but  the  principle  is  the  same. 

Seed-bags  are  bags  filled  with  flaxseed,  beans,  peas  or  wheat. 
These  bags  are  placed  at  the  bottom  of  the  hole,  preferably  in 


WATER,  THE  ENEMY  OF  THE  PETROLEUM  INDUSTRY  161 

some  clay  or  shale  formation.  The  bags  fit  the  holes  snugly  and 
are  rammed  down  with  the  drill  bit.  The  casing  is  then  let  down 
upon  the  seed-bags,  forcing  part  of  the  seeds  between  the  casing 
and  the  wall  of  the  hole.  The  seeds  expand  under  the  action  of 
the  water,  and  act  as  an  effectual  barrier.  Later,  the  bag  in- 
side the  casing  is  drilled  through.  In  some  cases  bags  are  tied 
around  the  casing  and  let  down  into  the  hole. 

Cementing  methods  have  become  very  popular  for  shutting  off 
water.  Several  different  methods  are  in  vogue.  One  requires 
that  the  casing  be  lowered  into  a  bed  of  cement  which  fills  the 
hole  to  some  distance  above  the  water  sand.  The  cement 
hardens  and  later  the  cement  inside  the  casing  must  be  drilled 
through  to  reach  the  oil  sand  below. 

Another  method  requires  that  the  casing  be  placed  in  the  hole 
and  raised  above  the  water  stratum  a  short  distance.  Cement 
is  gradually  pumped  into  the  hole  under  high  pressure  and  the 
casing  then  lowered.  As  the  cement  hardens  it  forms  an  effective 
barrier  to  water  entering  the  drill  hole  and  also  keeps  the  water 
from  coming  in  contact  with  the  casing  and  corroding  the 
same  Cementing  material  may  be  pumped  into  the  drill  holes  or 
may  be  dropped  in  by  means  of  bailers  or  tin  tubes. 

One  of  the  most  effective  and  also  the  simplest  methods  of 
shutting  off  water  sands  consists  first  of  drilling  10  or  12  ft.  into 
a  good  clay  or  soft-shale  bottom.  The  drill  hole  is  then  filled 
with  fine  mud  to  the  height  of  10  or  12  ft.  The  casing  islet  down 
on  the  bottom  of  the  hole,  the  fine  mud  settles  around  it  and 
when  later  drilling  is  continued  through  the  casing,  the  mud 
around  the  casing  forms  a  water-tight  joint  effectually  sealing 
off  the  water  stratum.  Generally,  however,  the  casing  is  driven 
10  or  12  ft.  into  a  shale. 

Various  other  methods  of  shutting  off  water  are  used  in 
different  parts  of  the  world,  but  the  several  methods  described 
above  are  the  principal  ones. 

Treatment  of  Secondary  Water  Troubles. — Secondary  water 
troubles  are  treated  in  a  manner  similar  to  primary  water 
troubles.  Where  the  casing  is  defective  it  must  be  withdrawn 
11 


162  PRACTICAL  OIL  GEOLOGY 

from  the  hole  and  replaced  with  new.  This  requires  re-cementing 
and  calls  for  the  methods  outlined  above.  Where  the  cement  in 
the  hole  is  at  fault  the  casing  must  also  be  pulled,  and  the  casing 
reset  and  re-cemented. 

Where  there  have  been  cave-ins  no  safe  method  of  procedure 
can  be  laid  down.  If  a  great  deal  of  water  enters  the  wall  it 
may  become  necessary  to  abandon  it.  This  is  true  especially 
where  there  is  but  a  single  oil  sand,  but  where  two  or  more  sands 
are  present,  the  zone  in  which  the  cave-in  has  occurred  may  be 
isolated  by  one  of  the  several  methods  used  to  shut  off  water 
sands. 

Prospect  holes,  especially  " wildcat"  holes,  when  carelessly 
finished  may  threaten  the  life  of  an  entire  field.  In  some  states 
such  holes  must  be  plugged.  The  law  specifies  the  size  of  plug 
and  the  amount  of  covering  above  it.  The  importance  of  taking 
proper  care  of  these  holes  must  not  be  underestimated.  The 
true  remedy  for  flooding  does  not  by  any  means  lie  in  the  treat- 
ment of  water  after  it  appears,  but  in  taking  every  possible  pre- 
caution to  see  that  the  casing  and  cement  are  intact  when  the  well 
is  first  brought  in.  Later,  accidents  may  happen  or  primary  water 
may  replace  the  oil,  but  these  causes  of  trouble  are  unpreventable. 

Cave-ins,  however,  can  be  prevented  by  using  strainers  or 
screens  in  place  of  the  perforated  casing  generally  used  in  the  fields. 
It  is  customary  to  cut  a  hole,  square  or  round,  and  with  dimen- 
sions anywhere  from  J^-in.  diameter  to  %-in.  These  large  per- 
forations admit  a  great  deal  of  sand  and  in  consequence  cavities 
form  around  the  casing.  Cave-ins  may  result,  due  to  the  cavi- 
ties, conditions  favoring  them,  and  much  damage  be  done  to  the 
well.  Strainers  would  do  away  with  cave-ins.  These  strainers 
are  made  by  cutting  horizontal  slots  in  the  casing  with  the  width 
of  the  slots  slightly  smaller  than  the  average  diameter  of  the 
sand  grains.  By  using  either  form  of  strainer  a  minimum  quan- 
tity of  sand  enters  the  well,  with  the  result  that  cavities  do  not 
form  in  the  oil  zone.  Were  strainers  or  screening  used  in  the 
California  fields  more  generally,  disastrous  cave-ins  would  soon 
cease. 


WATER,  THE  ENEMY  OF  THE  PETROLEUM  INDUSTRY  163 

Where  a  neighboring  well  is  at  fault,  cooperative  methods  of 
treatment  should  be  resorted  to,  unless  the  neighbor  is  unwilling 
to  remedy  the  trouble  in  his  wells.  If  such  is  the  case,  the  state 
inspector  should  be  called  in  and  the  repair  work  done  at  the 
expense  of  the  unwilling  neighbor. 

Protection  from  Water. — In  some  of  the  Californian  fields, 
notably  Coalinga  and  the  Sunset-Midway  district,  water  troubles 
have  been  actively  combated  by  the  operators  who  have  created 
protective  associations. 

Every  company  is  asked  to  furnish  logs  of  its  wells.  Careful 
contour  maps  of  the  field  are  made  from  the  logs,  and  the  water 
and  the  oil  strata  are  correlated.  Armed  with  this  knowledge  the 
operators  are  better  able  to  proceed  intelligently  in  shutting  off 
water. 

Changes  in  Well  Temperatures  Warn  of  Danger  from  Water. 
— A  very  ingenious  method  is  employed  by  operators  in  the 
Baku  oil  fields  of  Russia  to  determine  water  troubles.  In  these 
fields  the  waters  that  flood  the  wells  are  hot.  Every  day  the 
temperature  of  a  well  is  taken.  If  the  temperature  increases 
3  or  4°  it  is  known  that  water  trouble  threatens  the  well  and 
remedial  measures  are  at  once  taken. 

Mud-Laden  Fluid. — Mud-fluid  is  commonly  used  in  the  rotary 
and  circulator  systems  of  drilling,  but  the  application  of  the 
mud-laden  fluid  to  conserving  oil  and  gas  sands  is  compara- 
tively recent.  The  theory  of  the  application  is  briefly  given 
below.  The  details  of  its  use  is  best  covered  in  Bulletin  134 
of  the  United  States  Bureau  of  Mines. 

The  mud-laden  fluid  is  used  to  shut  off  gas  and  water  sands  by 
the  introduction  of  a  fluid  sufficiently  heavy  to  overcome  oil  or 
gas  pressures  that  will  not  be  overcome  by  an  ordinary  head  of 
water.  Also  the  finely  particled  mud  acts  as  a  medium  of  plug- 
ging off  oil  and  gas  sands  without  letting  the  water  enter  the 
sands  as  is  the  case  with  clear  water. 

Mud-laden  fluid,  as  defined  below,  has  properties  decidedly 
different  from  ordinary  water  or  from  that  of  a  super-  or  over- 
saturated  mixture  of  mud  and  water.  It  is  a  mechanical  mixture 
of  mud  and  water. 


164  PRACTICAL  OIL  GEOLOGY 

Mud-laden  fluid  consists  of  finely  divided  clay  or  mud  held  in 
suspension  in  water.  The  mud  must  be  of  such  consistency  that 
the  particles  will  float  in  the  water  and  will  not  settle  to  the  bot- 
tom when  standing.  The  clay  must  be  pure  and  not  sandy. 
The  specific  gravity  of  such  mud-laden  fluid  should  not  be  over 
1.32.  By  volume  it  consists  of  15  to  20  per  cent,  of  the  mixture, 
and  by  weight  30  to  40  per  cent,  of  the  mixture. 

Pure  water  may  temporarily  "drown"  or  deaden  a  gas  sand, 
but  in  time  escapes  into  the  sand  or  is  blown  out  of  the  hole  when 
the  gas  pressure  is  a  little  above  the  pressure  of  the  water  column 
especially  if  its  water  escapes  into  the  sand.  Mud-laden  fluid  is 
sufficiently  heavy  to  overcome  high  gas  pressures.  A  column  of 
mud-laden  fluid  1  ft.  high  with  a  cross-section  of  1  sq.  in.  and 
a  specific  gravity  of  1.32  exerts  a  pressure  of  0.573  Ibs.  per 
cu.  in.  as  against  0.434  for  pure  water,  or  573  Ibs.  per  1000  ft. 
as  against  434  Ibs.  per  1000  for  pure  water,  and  has  the  very  great 
advantage  of  not  escaping  into  the  sands,  nor  does  it  pack  or  settle 
around  the  casing  in  such  a  way  as  to  " freeze"  or  bind  tight  the 
casing.  Casing  standing  in  wells  for  5  years  has  been  pulled 
freely  from  the  mud-laden  fluid. 

Mud-laden  fluid  and  its  use  is  simply  a  very  efficient  method  of 
packing  off  gas,  oil,  and  water  sands,  in  place  of  cementing  and 
the  older  methods. 


CHAPTER  X 


NATURAL  GAS 

Definition. — Natural  gas  is  any  gas  formed  in  nature,  as  for 
example,  marsh  gas,  sulphur  dioxide,  carbon  dioxide  (the  "choke 
damp"  of  the  mines),  sulphuretted  hydrogen,  and  petroleum  gas. 

However,  as  generally  understood,  natural  gas  is  the  gas 
obtained  from  oil  or  gas  fields,  which  is  burned  in  our  homes,  and 
factories  in  place  of  gases  manufactured  from  coal  or  from 
petroleum. 

Geographic  Distribution. — At  present  the  main  producing  gas 
areas  are  in  the  Kansas,  Oklahoma,  Louisiana  fields,  in  the  Penn- 
sylvania and  West  Virginia,  and  in  the  California  fields. 

Composition. — Natural  gas  is  composed  principally  of  the 
hydrocarbons,  methane,  (CH4)  (commonly  called  marsh  gas), 
and  ethane-  (C2H6). 

Two  elements,  carbon  and  hydrogen,  are  the  chief  constituents 
of  all  natural  gas. 

The  analysis  of  four  typical  gases  are  presented  below.. 

TABLE  XIV 


5 

3 

H 

I 

§ 

0 

_o 

a 

8, 

c 
o 

•"a 

1 

1 

Sample,  where  taken 

| 

1 

£ 

1 

J-g 

1 

| 

^ 

W 

o 

2 

W 

6° 

H 

w 

CH4 

C2H6 

CO2 

N2 

H 

He 

B.T.U.     per 

cu.  ft. 

1.  Pittsburg,  Pa  

92.0 

3.0 

2.0 

3 

978 

2.  Midway,  Cal.  (Dry)  

92.0 



3.2 

0.3 

3.2 

.... 

998 

3.  Hogshooter,  Okla.  (Dry)  

94.2 



1.0 

4.8 

.... 

.... 

1003 

4.  Glen  Pool,  Okla.  (Wet)  

38.75 

61.10 

2.2 

.  .  .  . 

1551 

5.   Dexter,  Kansas                          .         ... 

14.85 

0.41 

82.7 

1.84 

Non- 

combustible 

Gasea  2  and  3  are  dry  gases.    Gas  No.  4  is  "wet"  or  casing  head  gas,  which  contains 
petroleum  vapors.    Gas  No.  5  is  peculiar  in  that  it  contains  82.2  per  cent,  of  Nitrogen;  and 

1.84  per  cent.  Helium. 


165 


166  PRACTICAL  OIL  GEOLOGY 

Origin  of  Natural  Gas. — The  origin  of  natural  gas  is  as  doubt- 
ful as  the  origin  of  oil.  It  may  be  of  organic  or  inorganic  origin, 
that  is,  formed  by  the  decomposition  of  animal  or  vegetable 
matter,  or  it  may  be  the  result  of  the  interaction  of  certain  chem- 
icals lying  at  great  depths  underground. 

Some  marsh  gas  is  certainly  due  to  decaying  vegetable  and 
animal  matter,  and  it  is  not  improbable  that  the  natural  gas  from 
petroleum  is  also  of  the  same  origin. 

Methane  is  the  most  stable  of  the  hydrocarbons.  It  is  highly 
probable  that  the  heat  breaks  up  the  more  complex  hydrocarbons 
to  form  methane,  so  that  gas  is  being  continually  formed  in  the 
Earth  wherever  petroleum  is  found. 

It  is  not  at  all  improbable  that  tremendous  quantities  of  gas 
are  formed  in  the  change  of  lignite  to  bituminous  coal,  and  from 
bituminous  coals  to  anthracite. 

However,  speculations  along  such  lines  lead  to  endless  contro- 
versy, and  for  all  practical  purposes  it  is  sufficient  to  note  the 
occurrences  of  natural  gas,  and  its  economic  importance,  leaving 
speculations  as  to  its  origin  to  the  ultra-scientists. 

Relation  of  Gas  to  Bituminous  or  Petroliferous  Matter. — 
It  is  interesting  to  note  that  gas  occurs  in  regions  where  lignitif- 
erous,  bituminous,  or  petroliferous  shales,  are  found.  The  gas 
generally  occurs  above  the  shales,  or  in  porous  sand  lenses  inter- 
bedded  in  the  shales. 

The  relation  of  commercial  accumulations  of  natural  gas 
to  shales  are  important,  as  one  would  not  care  to  locate  a 
well  in  regions  where  shales  carrying  organic  matter  are 
absent. 

Some  commercial  gas  had  occurred  in  glacial  drifts  in  Iowa, 
Illinois,  and  Kansas.  The  gas  was,  in  all  likelihood,  marsh  gas, 
resulting  from  the  decay  of  vegetation,  buried  at  shallow  depths. 
It  may  also  be  possible  to  utilize  the  marsh  gas  formed  in  the 
Gulf  Coast  regions,  where  the  marsh .  gas,  if  collected  above 
ground  would,  in  some  places,  have  a  distinct  commercial  value 
as  fuel  for  houses.  Tremendous  quantities  of  decaying  vegeta- 
tion occur  along  in  the  Coastal  plains,  and  the  utilization  of  the 


NATURAL  GAR  167 

marsh  gas,  given  off  by  that  vegetation,  is  only  a  question  of 
time. 

Stratigraphy. — Natural  gas  occurs  in  formations  of  the  most 
recent,  to  those  of  the  early  Paleozaic,  Ordovician  and  Silurian 
age,  In  fact,  gas  seems  to  be  found  wherever  decaying  vegeta- 
tion or  animal  matter  has  occurred.  Commercial  accumulations 
are,  however,  dependent  upon  the  trapping  of  the  gas.  The 
presence  of  natural  gas  in  the  older  beds  presupposes  earlier 
vegetable  or  animal  life,  unless  the  gas  has  migrated  from 
younger  beds.  The  fossil  evidence  bears  out  this  view. 

Commercial  Deposits  of  Natural  Gas. — Commercial  deposits 
of  natural  gas  are  dependent  upon  two  essential  factors: 

1.  A  source  of  supply  for  the  gas. 

2.  Proper  reservoirs. 

The  source  of  supply  has  been  treated  under  origin,  and  the 
reservoirs  are  the  same  as  under  structure  in  Chapter  IV. 

Migration. — In  some  cases  the  gas  has,  in  all  probability, 
migrated  from  lower  beds  to  higher,  due  to  unconformities,  or  to 
faulting,  as  shown  in  Figs.  27  and  28,  page  62. 

Gas  due  to  its  nature,  travels  wherever  an  opening  is  afforded, 
and  only  comes  to  rest  where  there  is  a  relatively  impervious 
covering  to  prevent  its  escape.  The  covering  may  be  a  compact 
limestone,  dense  shale,  or  closely  cemented  sandstone. 

Gas  generally  occurs  with  oil,  but  there  are  many  productive 
gas  sands  which  do  not  carry  oil.  These  are  called  "dry"  gas 
sands,  to  distinguish  them  from  the  gases  carrying  petroleum 
vapors. 

Gas  sands  occur  above  or  below  oil  sands;  and  we  also  found 
districts  where  no  commercial  oil  sands  are  obtained. 

Gas  Pressures. — The  pressures  of  gas  vary  in  different  fields, 
and  for  different  depths  of  gas  sands.  There  is,  however,  a 
remarkably  close  relation  of  the  gas  pressures  to  the  depths  of 
the  gas  sands,  below  the  surface  of  the  earth.  Especially  is  this 
true  in  the  Mid-continent-Kansas  and  Oklahoma  oil  and  gas 
fields,  and  in  West  Virginia. 

A  well  (see  Fig.  100)  1000  ft.  deep  will  have  a  pressure  of  400 


168 


PRACTICAL  OIL  GEOLOGY 


Ibs.;  one  1500  ft.,  600  Ibs.;  one  2000  ft.,  800  Ibs.;  or  approximately 
40  Ibs.  per  100  ft.  A  study  of  a  large  number  of  wells  shows  that 
the  pressure  is  approximately  equivalent  to  a  water  column  the 
height  of  the  well. 

Allowances  must  be  made  for  the  differences  in  specific  gravities 
of  the  water,  and  the  differences  in  elevations  between  the  water 
table  near  the  surface  and  the  elevations  near  the  collars  of  the 
wells. 

A  column  of  fresh  water  1  ft.  high  and  1  in.  square  exerts  a 
pressure  of  .434  Ibs.  per  sq.  inch.  Underground  waters  in  the  oil 


FIG.  100. 

and  gas  fields  have  specific  gravities  of  1.0  to  1.3,  which  would 
make  the  water  much  heavier. 

In  calculating,  however,  one  can  disregard  specific  gravities, 
and  simply  use  0.4  Ib.  This  lower  figure  allows  sufficiently 
for  the  loss  in  theoretical  head,  due  to  friction,  differences  in 
elevation  of  the  water  table,  etc. 

Allowing  for  such  irregularities  the  pressures  are  remarkably 
consistent ;  so  much  so  that  in  regions  like  Oklahoma  and  Kansas 
the  gas  companies  base  their  estimates  of  pressures,  and  also 
figure  the  weight  of  fittings  necessary  to  control  the  well  upon  this 
important  relation. 

For  example,  fittings  for  a  well  1000  ft.  deep  need  only  to 
withstand  400  Ibs.  pressure.  A  factor  of  safety  of  2  gives  800  Ibs. 

Such  fittings  will,  however,  be  unsafe  with  wells  3000  ft.  deep, 
where  there  is  a  pressure  of  300  X  40  =  1200  Ibs.  per  sq.  in. 


NATURAL  GAS  169 

Fittings  capable  of  resisting  only  the  lower  pressure,  would  be 
inadequate  with  the  higher  one. 

There  is  considerable  question  as  to  the  cause  of  the  pressure, 
but  there  is  no  doubt  that  the  rule  holds  with  but  few  exceptions. 
There  is  such  a  close  relation  between  the  head  of  water  and  the 
gas  pressure  that  the  writer  does  not  feel  inclined  to  disregard  the 
Artesian  theory,  especially  for  sands  which  are  exposed  to 
meteoric,  or  rain  and  snow  water. 

The  point  is  raised  that  if  water  is  holding  in  the  gas,  that  the 
water  should  replace  the  gas  when  the  gas  pressure  is  weakened. 
Such,  in  fact,  is  generally  the  case.  The  gas  is  withdrawn  very 
rapidly.  The  water  moves  more  slowly  through  the  pore  spaces 
of  the  sand,  due  to  minute  friction  and  adhesion,  with  the  result 
that  the  gas  pressures  are  reduced  for  a  time.  By  shutting  in  a 
well,  however,  the  gas  pressure  rises  again,  showing  that  hydraulic 
pressure  is  back  of  it. 

The  writer  fully  realizes  that  artesian  conditions  do  not  apply 
at  all  places.  At  Fort  Smith,  Ark.,  little  or  no  water  is  found  in 
the  gas  sand.  Pressures  of  but  145  Ibs.  to  280  Ibs.  per  sq.  in., 
instead  of  400  to  800  Ibs.,  are  obtained  at  depths  of  1000  to 
2,000  ft.  The  sands,  however,  are  nearly  free  of  water.  (See 
Carl.  D.  Smith,  Bulletin  541,  U.  S.  G.  S.).  The  writer  under- 
stands that  the  synclines  carry  gas  in  that  field. 

Maximum  Gas  Pressures. — Maximum  gas  pressures  of  1260 
Ibs.  are  reported  at  Midway,  Cal.,  by  R.  P.  McLaughlin, 
California  State  Bulletin  No.  69,  1915.— 1500  to  1700  Ibs.  are 
reported  in  Green  County,  Pa.,  by  I.  C.  White,  W.  Va.  Geolog- 
ical Survey,  Vol.  la. 

Cementation  as  the  Cause  of  Abnormal  Rock  Pressures.— It 
may  be  that  where  the  pores  in  a  reservoir  are  filled  with 
cementing  material  that  the  gas  pressures  would  be  much 
above  normal.  If  the  original  reservoir  were  reduced  in  size, 
due  to  cementation,  the  gas  would  necessarily  lie  in  a  more 
contracted  area  and  be  under  higher  pressure.  This  may  and 
probably  does  account  for  some  abnormal  pressures  in  gas  fields. 

Gas  Volumes. — A  few  calculations  will  give  some  estimate  of 


170  PRACTICAL  OIL  GEOLOGY 

the  capacity  of  rocks  to  hold  gas;  also  the  large  acreage  that  must 
necessarily  be  drained  by  gas  sands. 

Assume  as  a  unit,  a  sand  1  ft.  thick  containing  10  per  cent, 
voids,  or  pore  space  per  cubic  foot,  and  covering  1  acre. 

An  acre  foot  of  43,560  ft.  of  sand  under  the  above  conditions 
contains  43,560  X  0.10  =  4356  cu.  ft.  of  pore  space. 

If  this  space  is  filled  with  gas  at  atmospheric  pressure  it  would 
contain  4356  cu.  ft. 

Gas  volumes  are  measured  at  an  average  mean  atmospheric 
pressure  of  14.4  Ibs.  per  sq.  in.,  and  at  a  pressure  of  4  oz. 
(0.25  Ib.)  above  the  mean  atmospheric  pressure.  The  pressure 
approximately  corresponds  to  14.7  Ibs.  at  60°F.,  the  pressure  at 
mean  sea  level.  We  will  accept  the  14.7  in  our  estimates. 

Theoretically,  the  temperature  at  depth  should  be  taken  into 
account,  but  in  an  approximate  calculation,  like  that  below,  such 
refinements  are  of  little  value. 

The  number  of  expansions  for  100  Ibs.  "rock"  pressure 
would  be 

100  +  14.7       __ 

_ —      =  7.8  expansions. 

The  total  number  per  acre  foot  will  be  volume  of  gas  X  expansions 
=  4356  X  7.8  =  33,976  cu.  ft.  per  rock  pressure  of  100  Ibs. 

At  500  Ibs.  rock  pressure  the  expansions  are  35,  and  the 
volume  per  acre  foot  =  4356  X  35  =  152,460  cu.  ft. 

For  1000  Ibs.  pressure  the  expansions  are .  ,  „     -   =  69 

instead  of  7.8,  as  one  would  at  first  imagine,  the  added  14.7 
Ibs.  becoming  of  less  consequence  the  higher  the  rock  pressure 
becomes.  The  volume  will  be  4356  X  69  =  300,582  per  acre  ft. 

As  several  wild  gas  wells  have  produced  from  30,  to  70,000,000 
cu.  ft.  per  day,  and  kept  it  up  for  months  it  is  a  matter  of 
wonder  to  understand  where  the  gas  came  from. 

The  graphic  chart  (see  Fig.  101,  after  U.  S.  B.  M.)  gives  some 
idea  of  gas  volumes  in  producing  fields.  This  was  taken  at  the 
famous  Caney,  Kansas,  gas-field,  and  shows  the  life  of  that 
remarkable  field. 


NATURAL  GAS 


171 


The  high  average  volume  of  10,694,400  cu.  ft.  per  well,  with 
the  rock  pressure  of  490  Ibs.  per  sq.  in.,  shows  that  such  wells 
must  have  drained  a  very  large  area,  indeed. 


14,000.000  I  |'|  I  1  |'|    iji  i 

1  1  I  i  i  i  II  I  I  I  i  MI  700 

13,000,  000.;**-  + 
fa      12,000,000  - 

650 

i::;;;;;::;:;:    8 

coo  5 

H                  ::::::::::: 
g                ::::::::::: 

*      11,000,000  -- 

o 

g      10,000,000   

--^  

-  500  of 

d       9,000,000   -- 
p,       8,000,000  

a 

„       7,000,000  -- 

S                  ::::::::::: 

..........  ....4500 

iijiiiipiipui 

2       6,000,000  
^       5.000,000  -- 

ft       4,000,000  -- 
H                             

3     3,000,000  

\.o  . 

H-  «0  tO  CO 

S  8  i  8 

SB  ROCK  PRESSURE  PE1 

>     2,000,000  :: 

1,000,000  

100  3 

1  II  1  1  1  1  Mil  111,1   o 

1906 


1908  1909 

TIME.  YEARS 
FIG.  101. 


1910 


1911 


Petroleum  may  act  as  a  solvent  for  the  gas,  and  hold  tremend- 
ous volumes  in  solution.  Hydraulic  pressures  set  up  by  hydro- 
static head  are  certainly  not  sufficient  to  liquefy  the  gas. 

Spacing  of  Gas  Wells. — Gas  has  such  mobility  that  one  gas  well 
situated  at  the  top  of  a  dome  should  in  time  drain  all  the  gas  from 
the  sand. 

Good  practice,  however,  allows  100  acres  of  land  per  gas  well. 
One  well  per  40  acres  is  the  minimum  of  good  practice.  Allowing 
one  well  to  drain  100  acres,  a  well  under  the  conditions  given 


172 


PRACTICAL  OIL  GEOLOGY 


above,  would  take  care  of  152,460,000  cu.  ft.  for  a  pressure  of 
500  Ibs.  and  a  sand  10  ft.  thick. 

Heating  Value  of  Gas. — The  value  of  natural  gas  in  heat  is 
approximately  1000  B.t.u.  per  cu.  ft.  The  higher  the  per- 
centage of  such  hydrocarbon. gases,  as  ethane,  etc.,  the  higher 
the  number  of  heat  units.  This  is  clearly  shown  in  the 
analyses  in  Table  XIV. 

Example. — No.  4  carries  an  unusually  high  percentage  of 
ethane,  which  is  higher  in  heat  units  than  methane. 

In  comparison  to  coal,  petroleum,  wood,  or  producers  gas,  we 
have  the  following  table  giving  some  comparative  heat  equiva- 
lents based  on  B.t.u.'s. 


TABLE  XV 


Fuel 

B.t.u. 

Equivalent  in 
B.t.u.'s   of 
1,000,000    cu. 
ft.  gas 

Value 

Natural  gas,  per  cu.  ft  

1,000 

1,000  000,000 

@  lOc.  per  1,000  =  $100.  00 

Oil  gas  

850 

1,176,470 

@25c.  per    1,000  =  $294.  00 

Producer  gas,  per  cu.  ft  

200 

5,000,000 

@5c.    per    1,000  =  $250.  00 

Petroleum  14,  B.  gravity  
Coal  (bituminous),  per  Ib 

18,500 
12,500 

160  bbls. 
40  tons 

@$1      per     bbl.  =  $160.00 
@$2  50  per  ton, 

Wood  per  Ib  

5,000 

66.6     cds   (1 

40  tons  =  $100.  00 
@$3.50  per  cord  =  $198.  00 

cd.~  3,000  Ibs. 

However,  B.t.u.'s  do  not  measure  the  true  efficiency  of  gas. 
Combustion  of  gas  is  greater  than  the  other  fuels,  so  the  actual 
efficiency  will  be  10  to  20  per  cent,  greater. 

As  the  combustion  of  gas  is  superior  to  coal,  oil  or  wood,  the 
efficiency  of  gas  is  greater  than  the  heat  units  would  indicate. 

Casing  Head  Gasoline. — The  development  of  the  casing 
head  gasoline  industry  has  sprung  into  prominence  within  the 
past  five  years. 

It  has  been  found  that  gas  coming  directly  off  petroleum  carries 
hydrocarbon  vapors  that  can  be  condensed  by  pressure.  These 
vapors  consist  of  the  lighter  hydrocarbons  that  go  to  make  up 


NATURAL  GAS  173 

naphthas,  benzene,  and  gasoline,  having  specific  gravities  of  70° 
to  100°  B. 

A  rich  gas  may  carry  5  gallons  of  gasoline  per  1000  cu.  ft.,  but 
1  gallon  per  1000  cu.  ft.  can  be  made  to  pay  if  a  sufficiently  large 
quantity  of  gas  is  obtained.  The  gasoline  is  first  extracted, 
and  the  "dry"  gas  is  introduced  in  the  gas  line  for  fuel. 

CONCLUSION. — The  geology  of  natural  gas  is  readily  under- 
stood. The  chances  of  the  occurrences  of  commercial  natural 
gas  is  so  much  greater  than  other  fuels,  that  the  writer  has  won- 
dered at  the  relative  lack  of  interest  of  manufacturers,  in  the 
possibilities  of  employing  such  a  cheap  fuel,  and  also,  at  the 
lack  of  comprehension  by  gas  men,  of  the  value  of  geology  as  ap- 
plied to  prospecting  for  natural  gas. 


CHAPTER  XI 
CAUTIONS 

In  the  foregoing  chapters  the  elementary  principles  of  oil- 
field geology  have  been  outlined.  A  few  further  points  may 
be  of  interest. 

Prejudice  against  Geologists. — In  many  places  much  prej  udice 
exists  among  drillers  and  operators  against  geologists.  There  is 
absolutely  no  need  for  such  feeling.  Both  classes  of  men  have 
their  distinct  work  to  do.  A  driller's  work  supplements  that  of 
the  geologist,  who  blazes  the  way  into  new  districts  and  points 
out  the  best  places  to  drill.  Instead  of  conflict,  there  should 
be  mutual  respect  and  cooperation. 

Just  because  most  geologists  are  men  of  technical  training  and 
education  is  no  reason  for  prejudice  on  the  part  of  the  operator  or 
driller.  The  geologist  in  his  line  is  as  practical  as  the  driller,  and 
uses  perfectly  open  and  legitimate  methods  of  work,  and  his 
working  methods  and  instruments  are  as  well  standardized  as  are 
the  tools  and  methods  of  the  driller. 

Again  men  who  are  ignorant  of  geologists'  methods  of  work  and 
knowing  of  their  success,  will  overrate  the  geologist  and  his  liabil- 
ity to  make  mistakes,  and  also  in  some  cases  put  the  geolo- 
gist oji  a  pedestal.  When  he  fails,  as  is  often  the  case,  they 
lose  faith  in  a  geologist's  work.  Minimizing  risks  is  the  best 
work  of  a  geologist,  and  if  he  reduces  chances  on  wildcats  from 
one  in  ten  to  six  in  ten,  then  he  has  accomplished  good  work.  A 
geologist's  function,  however,  is  far  more  comprehensive  than 
locating  " wildcats"  as  one  will  clearly  understand  if  he  has  fol- 
lowed the  earlier  chapters  closely. 

Exactness. — Many  people  think  that  a  geologist's  work  is 
exact.  Such  is  not  the  case,  by  any  means.  Where  under- 

174 


CAUTIONS  175 

ground  conditions  are  to  be  considered,  exactness  ceases.  One 
cannot  definitely  say  that  a  certain  condition,  such  as  a  fault  or  a 
fold,  persists  underground.  Many  times  it  may  not  do  so.  One 
must  qualify  such  statements  by  saying  in  all  probability  such  a 
condition  may  exist  underground,  or  the  evidence  leads  us  to 
believe  such  a  condition  to  exist.  Depths  are  predicted  within 
certain  limits,  not  down  to  the  last  foot. 

The  work  of  the  geologist  is  not  exact  nor  can  it  be  so.  There 
are  districts  in  which  exposures  are  so  scarce  that  one  cannot 
obtain  a  sufficient  amount  of  data,  on  which  to  base  any  positive 
conclusions.  In  such  cases  one  must  leave  the  structure  un- 
mapped and  uncertain.  In  all  mapping  and  study  of  the 
structure,  the  percentage  of  errors  varies  due  to  the  following 
factors: 
I.  Engineering  errors: 

(a)  Instrumental  errors. 

(&)  Errors  in  observation. 

(c)  Errors  in  reducing  from  large  natural  scale  to  small  map 
scale. 

(d)  Errors  due  to  drill  logs. 
II.  Geological  errors: 

(a)  Errors  due  to  the  thickening  or  thinning  of  beds. 

(6)  Errors  due  to  unseen  faulting,  unconformities,  lensing,  etc. 

The  first  set  of  errors  are  mechanical  and  personal ;  the  second 
set  due  to  natural  factors. 

Instrumental  errors  can  be  avoided  by  careful  work.  Where 
an  aneroid  barometer  is  employed,  the  per  cent,  of  accuracy  will 
not  be  as  great  as  with  a  spirit  level  or  plane  table  and  alidade 
system.  Errors  in  observation  are  due  to  the  personal  equation 
and  are  only  overcome  by  careful  checking.  Errors  in  reducing 
from  natural  scale  to  map  scale  are  not  of  such  importance  if  the 
earlier  work  has  been  carefully  done. 

Errors  due  to  geological  conditions  such  as  the  thickening  or 
thinning  of  beds  and  to  unconformities,  faults,  etc.,  cannot  be 
predicted.  Such  errors  are  due  to  natural  conditions  that  occur 
at  depth  and  cannot  be  foreseen  by  the  geologist  or  engineer. 


176  PRACTICAL  OIL  GEOLOGY 

Errors  in  well  logs  are  the  results  of:  1.  Improper  classification 
of  strata.  2.  Inaccurate  measurements.  3.  Careless  log  making. 

Often  drillers  and  geologists  fail  to  classify  beds  properly.  A 
bed  that  may  in  reality  be  200  ft.  lower  is  sometimes  called  a 
certain  key  bed — say  the  Oswego  limestone  when  it  may  be  an 
entirely  different  bed. 

The  measurements  vary  with  the  methods  used.  The  usual 
way  is  to  measure  off  units  on  the  sand  line  or  steel  drilling  cable, 
wrap  a  string  around  the  place  of  measurement,  and  then  to  count 
the  number  of  strings  or  units  that  go  into  the  hole  when  the  sand 
line  or  the  cable  is  unwound.  The  unit  taken  with  the  sand  line 
is  the  length  of  line  from  the  top  of  the  sand  line  reel  to  the 
top  of  the  casing,  approximately  170  ft.  for  the  82-ft.  derrick. 
This  distance  is  sometimes  measured  with  a  5-ft.  stick,  though  a 
steel  tape  is  generally  employed. 

If  the  drill  cable  is  used  for  measurements,  the  length  of  line 
from  the  top  of  the  bullwheels  to  the  top  of  the  casing  is  used,  and 
the  measurements  of  the  unit  is  obtained  by  the  steel  tape  or  5- 
ft.  stick.  Obviously  when  measuring  with  a  5-ft.  stick — slips 
occur,  and  measurements  a  foot  short  or  long  per  unit  may  be 
obtained.  This  error  when  multiplied  by  ten  may  mean  an  error 
of  10  or  even  20  ft.  in  deep  holes. 

If  the  measurements  are  carefully  made  by  such  methods  the 
results  may  be  correct  within  a  "  screw  "  of  5  ft.  Closer  estimates 
are  useless  as  the  driller  tells  the  change  in  his  formation  by  the 
''feel"  of  his  tools,  the  sound,  etc.,  and  he  may  drill  several  feet 
into  a  stratum  without  knowing  it. 

Total  depths  are  accurately  obtained  by  measuring  with  a 
steel  line.  Where  the  casing  measurements  are  taken,  it  is  easy 
to  check  the  thickness  of  formations;  also  the  total  depth  is 
accurately  obtained  in  this  way. 

Careless  log  making  is  responsible  for  many  errors.  The  driller 
may  encounter  a  certain  bed  at  a  specific  depth.  Instead  of 
marking  the  depth  at  once  on  his  log  book  the  driller  may  carry 
it  in  his  head,  and  if  busy,  forget  the  exact  figures,  with  the  result 
that  he  later  approximates  the  depth  and  is  off  sometimes  20 


CAUTIONS  177 

ft.  or  more.  Such  errors  can  only  be  eliminated  by  carefulness 
on  the  part  of  the  driller. 

However,  even  eliminating  all  instrumental  and  personal  errors 
there  are  still  the  uncertain  geologic  factors  that  may  upset  the 
most  careful  surface  measurements.  Nicety  and  exactness  is 
desirable,  but  there  is  a  limit  to  refinement  of  work  beyond  which 
it  is  a  waste  of  time  to  go. 

Salting  Samples. — "Salting"  is  sometimes  resorted  to  as  in 
mining.  A  few  examples  of  such  methods  will  prove  instructive. 
The  writer  has  in  mind  one  company  that  was  accused  of  em- 
ploying a  "salting"  method.  The  true  solution  in  this  case, 
however,  was  far  different,  and  the  "salting"  was  unconsciously 
done  by  the  examiner  himself.  The  consulting  engineer  who 
made  the  examination  used  what  is  popularly  called  a  "bucket 
gauge."  This  method  of  gauging  consists  in  holding  a  5-gallon 
bucket  at  the  mouth  of  the  lead-line  and  noting  the  time 
required  to  fill  the  same.  A  number  of  important  factors  must 
be  taken  into  account  in  figuring  production  by  this  method.  Oil 
shrinks  from  20  to  60  per  cent,  due  to  the  escape  of  enclosed 
gas.  It  is  customary  to  allow  the  oil  to  settle  24  hours  and 
then  measure  the  shrinkage.  Also  to  have  an  accurate  gauge  the 
same  rate  of  pumping  speed  must  be  maintained.  Water  and 
sand  are  also  present  in  many  cases.  Where  from  10  to  15  or 
even  50  per  cent,  of  water  may  occur,  precautions  must  be  taken 
to  provide  against  mistakes  in  not  testing  for  same.  The  con- 
sulting man  in  the  case  under  consideration  was  ignorant  of  these 
elementary  principles,  and  reported  upon  production  as  he 
found  it.  As  a  result  his  report  showed  a  production  100  per 
cent,  greater  than  the  actual  production.  The  purchasers  com- 
plained that  they  had  been  cheated,  yet  their  expert  sent  in  a 
true  report  according  to  his  light  on  the  subject.  No  one  at  all 
familiar  with  oil-field  methods  would  have  considered  such  a 
gauge  as  accurate.  The  best  method  is  to  obtain  reports  for 
runs  made  to  pipe-line  companies,  and  then  check  up  the  same 
by  extended  tests  at  the  wells,  using  both  bucket  and  tank  gauges. 

The  only  accurate  gauge  method  is  an  actual  tank  gauge  showing 
12 


178  PRACTICAL  OIL  GEOLOGY 

a  run  over  a  period  of  a  week  or  more,  with  the  examiner  or  his 
representative  on  the  ground  at  all  hours.  Wells  vary  greatly 
in  their  production  from  hour  to  hour,  but  a  24-hour  gauge  will 
be  fairly  constant  provided  the  pumping  speed  is  the  same. 

Prospect  holes  are  sometimes  salted  simply  by  filling  with  oil 
and  then  bailing  out  the  same  for  prospective  investors.  Such  a 
method  is  crude  but  has  been  successful  during  "boom"  times. 

A  very  interesting  and  yet  amusing  incident  occurred  to  a 
friend  of  the  writer  who  undertook  an  examination  in  Louisiana. 
An  oil  spring  was  reported  to  this  friend  and  he  made  a  trip  to  the 
region  in  which  the  find  was  reported.  There  was  undoubtedly 
oil  in  the  spring  as  a  thick  scum  of  heavy  oil  floated  on  the  surface 
of  the  water  and  more  bubbled  up  from  below.  The  consulting 
man  studied  the  spring  which  lay  at  the  bottom  of  a  gentle  slope. 
The  district  was  one  that  showed  little  or  no  evidence  of  struc- 
tural deformation  and  according  to  all  reports  on  the  district, 
oil  should  not  have  been  found  if  structure  counted  at  all.  But 
there  was  the  oil  spring  to  confute  all  theories.  The  engineer, 
however,  felt  that  something  was  wrong.  As  the  region  was  a 
wild  one  and  the  natives  a  hard  lot,  the  owner  of  the  spring  in 
particular  having  the  reputation  of  being  a  "killer,"  the  young 
geologist  said  little  and  sparred  for  time.  That  night  he  stole 
from  bed,  located  a  pick  and  shovel,  and  started  for  the  spring, 
some  half  mile  from  his  host's  house.  On  reaching  the  spring, 
queer  to  relate,  no  oil  bubbled  up.  Getting  down  on  his  hands 
and  knees  he  probed  around  the  bottom  of  the  spring;  his  hand 
encountered  a  small  pipe  which  gave  a  clue  to  the  solution  of  the 
mystery.  There  was  but  one  place  the  oil  could  come  from,  and 
that  was  near  the  top  of  a  small  knoll  close  by.  The  engineer 
finally  located  a  spot  that  showed  signs  of  having  been  recently 
dug  up,  and  soon  uncovered  a  large  barrel  containing  crude  pe- 
troleum, a  "plant"  that  might  have  made  the  owner's  fortune. 
The  engineer  covered  up  all  traces  of  his  visit  and  returned  to 
bed  and  next  morning  took  the  first  stage  out  of  the  region  and 
did  not  return  to  buy  up  the  promising  oil  land. 

Occasionally  samples  of  oil  are  switched  and  fraud  thus  per- 


CAUTIONS  179 

petrated  on  the  examining  man.     Careful  attention  to  samples 
will  do  away  with  such  fraud,  however. 

Self  Deception. — Self  deception  as  regards  oil  properties  is  not 
unusual.  Thus  in  drilling  for  oil  some  of  the  lubricating  oils 
for  the  machinery  have  been  known  to  leak  into  the  drill  hole, 
causing  a  temporary  excitement,  as  the  drillers  thought  the  traces 
of  oil  presaged  a  good  supply  below  and  boomed  the  district  on 
such  showings.  If  oil  has  been  found  in  quantity  it  nearly 
always  shows  strongly  in  the  material  dumped  from  the  bailer,  and 
a  study  of  the  sand  pile  will  generally  give  a  good  clue  as  to 
whether  or  not  oil  has  been  found.  However,  the  only  true  test 
lies  in  a  careful  examination  and  actual  gauging  of  the  well.  An 
actual  pumping  test  tells  the  tale  and  no  engineer  should  be 
content  without  making  such  a  test  where  it  is  possible  to  do  so. 


INDEX 


Accumulation,  compression  in,  7 

folds  favorable  for,  49 

of  natural  gas,  167 

of  petroleum,  5 

of  sediments,  31 

water  in,  7 

Acreage,  favorable  in  proven  areas, 
134 

in  "wildcat"  areas,  11 

per  well,  138 

Ages  of  formations,  4-2,  46 
Amount  of  oil,  134 
Animal  theories,  3 
Anticlinal,  domes,  49,  50,  57,  100 

theory,  6 
Anticlines,  49 

assymmetrical,  95 

classification  of,  49 

compound,  51 

curving  axis  of,  106 

definition  of,  51 

faulted,  81 

method  of  estimating  depths  on, 
106 

pressure  in,  120 

plunging,  48,  91 

steep  dips  of,  102 

symmetrical,  95 

twin,  99 
Axis,  definition  of,  50 

shifting  of.  51 

B 

Bailer,  158 
Baume  system,  14 


181 


Bitumen,  17 

Boulders,  effect  on  drilling,  119 


Cables,  size  of,  116 
Capillarity,  8 
Capping  of  oil  strata,  12 
Carbide  theory,  1 
Casing,  accidents  to,  154 

collapsing  of,  123 

sizes  of,  112,  140 

table    of    collapsing    pressures 

of,  123 

Casing-head  gasoline,  172 
Cave-ins,  cause  of,  154 

prevention  of,  162 

results,  157 
Cavities,  classification  of,  120 

effect  on  casing,  126,  154 

on  drilling,  98,  120 
Circulator  method  of  drilling,  88 
Clay,  definition  of,  31 

effect  on  drilling,  118 

on  topography,  74 
Coal,  beds,  drilling  through,  124 

theory,  4 

Complete  oil  analysis,  28,  29     . 
Compound  anticlines,  51,  99 
Compression  due  to  rock  pressure, 

121 

Conformity,  definition  of,  33 
Conglomerate,  definition  of,  30 

effect  in  drilling,  118 
Contact  lines,  88 
Convergence,  amount  of,  87 

correction  for,  86 

definition  of,  82 


182 


INDEX 


Convergence,  map  of,  82 
Correlations,  fossils  in,  36 

lithologic  similarity  in,  32,  35 
Cross-sections,  use  of,  91 
Crooked  holes,  114 
Curved  axes,  effect  of,  107 


Defensive  tactics,  methods  used,  144 

Deformations,  97 

Deposition,  31 

Depths,  calculation  to,  103 

effects,  on  choosing  rig,  108 
of  faults  on,  62,  63 
of  steep  dips  on,  101 
of  topography  on,  96 
on  pressure  of,  122 
on  size  of  cable,  116 
on  size  of  casing,  113 
on  time  of  drilling,  115 
of  formations,  96 
Derricks,  heights  of,  116 
Diatom  theory,  4 
Dip,  calculation  of,  103 
definition  of,  91,  92 
effect  of  steep  dips,  102 
Dip  slopes,  97 
Disappearance  of  folds,  99 
Domes,  anticlinal,  49,  50,  57,  100 
classification  of,  49 
definition  of,  57 
description  of,  59 
importance  of,  58 
locating  on,  100 
mapping  of,  81 
Drainage,  areas,  11,  75 

of  properties  by  neighbors,  143 
of  sands  by  erosion,  97 
Drillers,  work  of,  174 

errors  of,  175 
Drilling,  cables  in,  110 
crooked  holes  in,  114 


Drilling,  derricks,  heights  in,  116 

effect  of  cavities  in,  120 
of  coal  beds  on,  125 
of  formations  in,  95,  117 
of  rock  pressure  on,  120 

factors  in,  108 

in  California,  112 

in  Oklahoma,  112 

logs  in,  125 

objects  in,  108 

rigs,  choice  of,  108 

rotary,  113 

size  holes,  112 

systems  of,  108 

time,  140 
Dry  sands,  effect  on  drilling,  117 

gas  escape  into,  148 

loss  of  production  in,  148 

spots,  reasons  for,  139 
Dynamic  heat,  13 


E 


Earth  curves,  description  of,  48,  92 
Effects  of  compression,  on  migration, 
7,  16,  17 

on  specific  gravity,  15 
Effects  of  compression,  on  structure, 

49,  50 

Elemental  analysis,  22 
Erosion,  effect  of,  97 

features  of,  97,  98 
Errors,  due  to  logs,  176 

due  to  observation,  177 

engineering,  177 

geological,  177 

instrumental,  177 
Expansion  of  natural  gas,  170 


Faults,  classes  of,  60,  62 
definitions  of,  60 


INDEX 


183 


Faults,  effect  on  depth,  to  oil,  62 
on  dip,  62 

in  drilling,  120 

in  locating,  97 

in  production,  139 

in  saline  domes,  59 

mapping  of,  81,  83 

normal,  61 

reversed,  61 

strike,  62 

Flooding,  by  water,  139 
Folding,  anticlinal  type  of,  6 

disappearance  of,  99 

heat  generated  by,  13 
Formations,  definition  of,  40 

effect  on  drilling,  117 

oil  formations,  or  "sands,"  42, 

47 

Fossils,  application  of,  36 
Fractional  analyses,  23,  27 


G 


Geological  pressures,  167 
Geologic  column,  41 

distribution  of  natural  gas,  165 

eras,  table  of,  41 

names,  39 

use  of,  38 

Geologists,  prejudice  against,  174 
Grades,  table  of,  93 
Graphic  method  of  finding   depths, 

105 
Gravel,  definition  of,  30 


IT 


Hade,  107 

Hardness  of  strata,  117 
effect  on  drilling,  117 
effects  in  shooting  of,  124 

Heating,  effects  of  natural  gas,  79, 
172 


Heating,  effects  of  oil,  22,  172 
Hydraulic,  drill  bits,  118 

pressure,  123,  164,  168 

rig,  109 

rotary  system,  108 


Igneous  rocks,  definitions  of,  30 
sedimentaries  derived  from,  32 
topography  of,  77 

Impervious  beds,  effect  of,  12 

Indications  of  oil,  73 

Inorganic  theories,  1 

Isocline,  51 


Joint  planes,  effect  of,  100 
wells  in,  100 


K 


Key  wells,  146 


Lakes,  well  locations  in,  101 
Land  plant  theories,  3 
Lenses,  effect  on  dry  holes,  65 

oil  accumulations  in,  66,  67,  68 
Limestone,  as  reservoirs,  9 

definition  of,  31 

effect  on  drilling,  118 

gypsum  and  water  theory,  2 

on  topography,  74 
Lithologic  similarity,  35 
Logs  (records),  124 

errors  in,  175,  176 

form  for,  126 

importance  of,  79 

in  production,  128 

in  water  troubles,  157 


184 


INDEX 


Logs,  interpretation  of,  126,  127,  128 

mapping  from,  125 

value  in  drilling,  125 
Losses  in  extraction,  137 


M 


Map,  construction  of,  83 

contour  of  terrace,  56 

geologic,  88 

of  faulted  anticline,  81 

practical  application  of  contour, 
148,  149 

sand,  67 

structure,  78 

symbols  for,  89 
Mapping,  results  from,  72 
Measure  of  rock  pressure,  122 
Measurements,  errors  in,  176 

of  dips,  91 

of  drill  holes,  130 

of  plunge,  91 
Member,  definition  of,  40 
Migration,  definition  of,  15 
Minor  folds,  99 
Models,  use  of,  91 
Metamorphics,  definition,  of,  30 
Method  of  estimating  well  depths, 

130 
Mud,  fluid,  163,  164 

effects  of,  15 


N 


Names  of  oil  sands,  42,  43,  44,  45, 
46 

Natural  gas,  analysis  of,  165 
composition  of,  17,  20 
casing-head  gasoline  from,  172 
cementation  in  pressure,  169 
commercial  deposits,  167 
heating  value  of,  172 
main  producing  areas  of,  165 


Natural  gas,  maximum  pressures  of, 

169 

migration  of,  167 
origin  of,  166 
pressures  of,  167 
relation  to  bituminous  matter, 

166 
stratigraphic     distribution     of, 

167 

wells,  spacing  of,  171 
volumes,  170 


Oceans,  well  locations  in,  101 

Offensive  tactics,  140 

Offsetting  wells,  144,  145 

Oil  fields  on  flanks  of  mountains,  48 

Oil  sands,  names  of,  42,  43,  44,  45, 

46 
Oil  signs  of,  70 

well  screens,  types  of,  141 
Olefines,  table  of,  21 
Operating    problems,    classification 

of,  139 

Organic  theories,  2 
Origin  of  natural  gas,  166 
Origin  of  petroleum,  1 
Overlap,  35 


Paraffines,  table  of,  20 
Petroleum,  accumulation  into  com- 
mercial deposits,  5 
analysis  of  California,  23 
of  Gushing,  Okla.,  29 
of  Kentucky,  25 
of  Oklahoma,  24 
of  Pennsylvania,  26 
of  Texas,  26 
of  Wyoming,  27 
chemical  analysis,  22 


INDEX 


185 


Petroleum,  chemistry  of,  17 

classification  of  theories,  1 

commercial  analyses  of,  24,  25, 
26,  27,  28,  29 

complete  analysis  of,  28 

composition  of,  17 

compression  in  accumulation  of, 
7 

formation  of,  4 

indications    of,    favorable,    73 
unfavorable,  73 

inorganic  theories  of,  1 

migration  of,  15 

organic  theories  of,  3 

origin  of,  1 

reservoirs  for,  9 

saturation  of  sands,  10 

specific  gravity  of,  14 

theories  of,  1 
Photography,  88 
Plug,  159 

Plunge,  definition  of,  93 
Porosity,  amount  of  oil  due  to,  134 

in  accumulation,  8 

in  reservoirs,  8 

in  saturation,  10 

pressure  and,  11 
Portable  rig,  108,  109 
Pressures,  gas,  167 

hydraulic,  124,  164,  168 

rock,  121,  169 
Production  curves,  137 
Production    estimates,    amount    oil 
in,  134 

losses  in,  137 

methods  of  obtaining,  134 

production  curves  in,  137 
Prospect  holes,  locating  of  test,  94 

results  from,  96 

water  in,  162 
Prospecting,  cautions  in,  70 

points  to  note  in,  69 

results  of,  72 


Quantity  of  oil  in  sands,  9 
R 

Rock  pressure,  cementation  as  cause 
of,  169 

compression  due  to,  121 

effect  in  collapsing  casing,  120 
Rocks,  classes  of,  30,  69 

conformity  of,  33 

deposition  of,  31 

igneous,  30,  69 

metamorphic,  30 

sedimentary,  30 

source   of   supply  of  material, 
32 

test  for  oil  in,  71 
Rotary  drilling,  109 


Saddles,  51 

Saline  domes,  5,  60,  100 
Salting  of  wells,  methods  of,  177 
Sands,  as  reservoirs,  9 

cementation  of,  35 

changes  in  character  of,  67 

deposition  of,  31 

dry  spots  in,  139 

effect  of  drilling  of,  117 

losses  in,  137,  138 

map,  67 

names  of  oil,  42,  45 

quantity  of  oil  in,  9,  134 

reservoirs  for  oil,  8 

saturation  of,  9 

test  for  oil  in,  71 

topography  affected  by,  74 

unknown,  142 

variation  in,  32 

water  in,  151 


186 


INDEX 


Sandstones,  definitions  of,  31 

effect  on  drilling  of,  117 
on  topography  of,  74 
Sand  map,  67 
Seaweed  theory,  3 
Sedimentary  rocks,  30 
Seepages,  as  indications,  69,  70 

locating  near,  94 

mistakes  for,  70 
Self-deception  in  drilling,  179 
Shale,  definition  of,  31 

effect  on  drilling  of,  118 

on  vegetation  of,  76 
Shooting  wells,  124 

acreage  per  well,  138 

effect  of,  124 

reasons  for,  124 
Specific  gravity,  14 

definition  of,  14 

economic  aspect  of,  14 

table  of,  15 

Speeding  wells,  reasons  for,  144 
Spots,  65,  66 
Standard  cable  tool,  drill  bit,  118 

rig,  109 

system,  108 
Stereograms,  use  of,  91 
Strainers,  use  of,  162 
Stratigraphic  distribution  of  petro- 
leum production,  53 

of  natural  gas,  167 
Stratigraphy,  definition  of,  30 
Strike,  definition  of,  92 
Structural  geology,  48 
Structural,  "highs,"  51 

"lows,"  51 
Structure    contours,   application   of 

maps,  148 
'  kinds  of,  79 

Superposition,  order  of,  35 
Swedge,  159 
Symbols,  map,  89 
Synclines,  classification  of,  49 


Synclines,  definition  of,  51 
oil  possibilities  in,  51 


Terrace,  description  of,  56 

locations  on,  56 
Test  for  oil  in  rocks,  71 
Time  in  drilling,  115 
Topography,  definition  of,  74 


U 


Unconformities,     angular,     example 

of,  33 

definition  of,  33 
effects  on  mapping  of,  176 
on  migration,  139 
on  well  locations,  64 
erosional,  example  of,  33 
Underground  structure,  48 
Underreamer,  use  of,  118 
Unknown  oil  sands,  142 
Uplifts,  48 


Valleys,  synclinal,  76 

Variation  in  beds,  32 
cementation  of,  33 
lateral  changes  in,  32 

Vegetal  theories,  3 

Vegetation,  79 

Vertical  sections,  37 

Voids  in  formations,  8 

Volcanic  domes,  49,  57,  100 
theories,  12 

W 

Water,  areal  limits  of  sands,  133 
as  an  oil  indicator,  73 
avoidance  in  locating,  99 


INDEX 


187 


Water,  changes  of  oil  sands  to  water 

sands,  133 

decreased  by  key  wells,  146 
drainage  of,  97 
effect  in   classifying  materials, 

125 

on  cavities,  120 
flooding  by,  139 
in  accumulation  of  oil,  5,  7 
in  anticlinal  theory,  6 
in  cementation  of  sediments.  33 
in  deposition  of  sediments,  30,  31 


Water,  in  drilling,  88,  108,  109,  113 
in  erosion,  70,  97 
in  formation  of  oil,  4 
in  migration  of  oil,  7,  12 
in  minor  folds,  99 
in  ocean  or  lake,  101 
in  oil  production,  132 
on  clays  and  shales,  118 
on  drilling  lines,  115 
on  gauging,  177 
on  rock  pressure,  123 
on  specific  gravity  of  oil,  14 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 
This  book  is  DUE  on  the  last  date  stamped  below. 


1947 


31lMaf90C$ 


LD  21-100m-12,'46(A2012sl6)4120 


YB  53779 


M100495 


THE  UNIVERSITY  OF  CALIFORNIA  LIBRARY 


